stares at the kid in a threatening position with ears to the side having an upset look in her eyes, her ears won't be to the back like she does when she is scared, it looks totally different from when she's scared
Link about cat body language, ears to the side is the same as ears to the back, at least in the sense that it communicates fear and/or anxiety (which is essentially the same underlying emotion).
From experience with cats that dislike each other, ears to the back is active fear (e.g. of being attacked), whereas ears to the side is more a sign of submission (basically saying "I don't want this conflict, I yield", which can occur without feeling afraid).
If they separate, it would be a different case. She'll just scurry and find a hiding. She runs when she hears the mother coming and she becomes afraid of the kids again as if she knows the mother will come to help her kids.
Based on what you've told us, I would infer that it is the mother who's been aggressive (or at least the cat thinks of her as aggressive), and the other cat has learned to avoid her (and by extension her children).
We have two 2 year olds and their mom to spend some time living with us.
I infer that the other cat lived here before the mom and kids? Their presence in what the cat considers its home (cats are territorial) may be enough for her to consider the mother (and by extension her children) as a threat.
But as long as the mom is not there she will try to avoid the kids and if necessary scare them. It's kind of crazy how one minute she is very confident and scaring the kids off, and then the next minute when the mom comes she is afraid to challenge the kids.
It's not that crazy. She knows that she can avoid the kids (and tell them to back off), but she expect the mother not to put up with that. It's similar to how a teenager has no issues with annoying or harassing their younger sibling (telling them what to do etc), but behaves when the parents are nearby.
I'm not sure if this is considered aggressive or not because the cat never tries to hit them. She never growls and only once hissed at a kid. She is able to growl though. She once did it to a 10 year old guest to kick the girl out of the room. It worked.
Nothing you've mentioned indicates aggression. Everything you listed can be explained by two cats who are both apprehensive of the other, and their defensive reaction is interpreted by the other as an intent to do harm.
There's no malevolence here.
A cornered animal is aggressive, but it's important to realize that this aggression is defensive in nature. The cat isn't attacking because it wants to attack, but rather it's retaliating against someone who it feels threatened by.
So let me play devil's advocate here:
- The other cat is suddenly faced with new inhabitants of its territory.
- The mother is naturally apprehensive of another cat, because she has babies to take care of.
- The mother chooses the safest approach and gets the other cat to back off just to be sure. Because it's for the safety of her children, she's a bit more... insistent on getting the other cat to back off (hormonal responses are strong responses).
- The other cat, already apprehensive of this territorial invader, now also sees a hostile reaction from the mother. So the cat makes the most logical inference: this is a hostile invader.
- The other cat detests this hostile invasion, but it knows that it can't win in a fight (most animals know that mothers are fierce, even if different species)
- The other cat knows that it can win against the children, as long as the mother doesn't get involved. Therefore, it tries to retain its territory from the children if the mom isn't nearby.
- The mother sees this unkind behavior towards her children and develops a perpetual dislike of the other cat.
- Both cats are now locked in a never-ending spiral of hostility, even though both of their apprehensive approaches were understandable and arguably justified.
This isn't easy. It's possible that a lot of damage has already been done, and will take a lot of time to undo these wrong first impressions.
I experienced a similar thing not too long ago. We have two sister cats (15mo now); who are incredibly shy and nervous. We added a third cat, less than half their age (6mo now), who is one of the most extroverted and happy cats I've ever encountered. The girls didn't like him at first, because he infringed on their serene atmosphere, was much too energetic and didn't observe social boundaries.
Essentially, you need to create some team building exercises. Put the cats in situations where they are situational allies, so they'll start to understand that they are not mortal enemies and can be friendly to one another.
The easiest one is breaking bread. In other words, communal feeding time. Everyone is hungry, everyone is being fed, everyone is happy. No one stole each other's food, there was enough to go around.
If the cats don't trust each other at the moment of being near each other to eat, try and do it in a way that they at least see each other even if there is a separator in-between.
Temporarily overfeed them if you must. It's important for them not to feel like they're fighting over a limited resource. There needs to be an abundance, so that there is no feeling of infringing on each other's meal.
It won't kill them to overeat a handful of times. If anything, it might make them more complacent (arguably lethargic), which makes them less likely to act out because they don't have the energy/willpower for it. Happy and satiated cats are less likely to pick a fight.
Obviously don't keep overfeeding them, but make sure that their reduced meal size does not create friction between the cats.
You can also share affection. One pet for mom, one pet for the other cat, one pet for kitten 1, one pet for kitten 2, repeat. There are a few benefits here:
- It repeats the "breaking bread" principle: everyone gets love from the same hand, everyone is happy.
- It subtly starts swapping smells between the two groups, because you use the same hand to alternate between them.
- If you give full attention to one cat, and the next day give full attention to the other cat, they won't really understand that you're dividing it fairly, because they don't observe the long term average. But by using the childish "one for A, one for B" system, it becomes much more apparent for the cats (who are no smarter than a human child) that there is fairness to the system, they are clearly equal to you.
Allow the cats to showcase themselves.
Our male kitten was eager to play with the laser but very haphazard in his approach. One of the girls loves the laser too, and has lightning reflexes (like this, but lightning fast).
So I intentionally allowed her to show off her skills. I started to move the laser too erratically for him to track it, but she was able to jump in and still expertly chase it. He was clearly impressed by that, because ever since then, he has voluntarily shared playtime with the laser, instead of always putting himself first. By now, he is also trying to do it her way, which is really cute to observe :)
For the other cat, I think you'll want to showcase how much the mom cares for her children. It humanizes (felinizes?) her, and should make the other cat realize that she's not being hostile, she's just defending her children.
For the mother, I think you need to show that the territory used to be the other cat's. She needs to realize that she's essentially a guest (as far as the other cat is concerned).
If one of the cats is notably more energetic than the other (which causes social friction), tire them out. The male kitten was much too playful for our serene girls, so I took him to another room and played with the laser until he was exhausted (actually panting). Their interactions were much smoother then, and the girls were much more resilient to see him become energetic after a while, instead of them starting off with him at maximum energy level.
Another thing, though this may appear cruel (but it isn't), is to not intervene in any conflict unless one of them is clearly yielding but being blocked from retreating. As long as both are willing participants in the conflict, that means they have some differences to settle.
If you stop the conflict dead in its tracks, all you've done is postpone it to a later date. You're now stuck with having to continually stop the same conflict whenever it happens.
Instead, you should try to become the referee. Don't stop the conflict, merely observe whether both participants are willing.
This is one of the things I regret doing to our male kitten. Whenever he playfully approached one of the girls (sleeping, or lying somewhere in peace), I immediately told him off and blocked him from disturbing her. I needed to do this >15 times a day.
When I was too sick to get up, I saw the same happen. I observed instead of intervened (since I assumed the same would happen when we weren't home anyway) The first three, four times, the girl was unfairly driven away from her cozy spot. The fifth time, she savagely swatted him (no claws, but a flurry of paw punches), and he immediately backed off and did not try again. One of the girls has a habit of being patient but also exploding (again, claw-free, which I'm grateful for) once her patience is gone, and he has learned to not poke that particular bear anymore.
The last point is arguably the most important:
Cats are territorial. Currently, the two parties are unable to share the territory because they do not trust each other. Both of them are also afraid that the other is trying to claim the entire territory, which sows the seeds of mistrust.
So divide the territory. Buy enough cat toys/scratching posts/beds to go around, so there's no conflict about having to share. Try to segregate their belongings, so that each cat has a personal space that is theirs. They're allowed to roam around the house, but don't make them sleep next to each other, because neither of them will feel at ease.
Over time, once the cats open up to each other, they'll realize that they can share the whole space, instead of each owning half of it.