My cockatiels stop feeding their chicks after about 5-6 days. Sometimes they feed them until they are 10 days old and then they'll stop. I have lost so many chicks due to this bad habit. What should I do make them keep feeding their chicks until they're fully grown?
There is a delicate balance between overseeing your cockatiels' hatchlings, and causing them stress. If the parents get stressed, or feel like they're in danger, they will stop caring for (even kill) their babies. Usually they will do this when they're still eggs, like if the breeder is looking into their nest too often, but it's possible that something is happening in the couple days that the babies have hatched where the parents don't feel safe caring for them anymore.
Another reason could be food. The link for Cockatiel Cottage explains that part. You need to be feeding your birds double what you normally would, plus regularly offering them soft food. That way they have enough for them, and the babies. If they don't have enough food for both, they will keep the food for themselves. Although there's the delicate balance again where you don't want them overfeeding the babies, as their crops need to be emptied every 24 hours, or they risk impaction.
The only other reason I can think of, is that there is something wrong with the babies, or they think there is something wrong with the babies. Birds can tell when their chicks are born with defects, or are too underdeveloped to survive. So it they don't think the babies will make it, they'll simply ignore them to focus on the ones that will. In this case, since all of the chicks are being ignored, there is something bigger going on.
I would suggest reading up on breeding cockatiels. Breeding any animal is a delicate process and should not be done without proper care. And since I do not specialize in birds, there is a great deal more you can find out from people other than me.
I'm hoping the couple you're breeding aren't related, as trying to breed parents that are related to each other produces nothing but chicks with birth defects, in which case the parents would ignore them.
I would do some research into crop impaction, as that's my best guess as to what's happening here. If they're being overfed, fed the wrong foods, the temperature isn't right, they have an infection, then their digestive systems will shut down. That would cause the parents to ignore them.
Finally, I think the best thing to do is get someone who breeds cockatiels, or, even better, a vet to take a look at both the adult pair, and the babies. To make sure that there isn't any underlying issues that is causing this. Ultimately I can give you all the advice in the world, but I cannot see the birds, so I'm inevitably missing something. Being able to see them helps a great deal.
Ultimately, you're probably going to have to take the babies yourself and hand-feed them sooner than you would normally. The links I provided should help you with that as well.