I have 2 one-year-old kittens and an old dog who we get groomed (shaved) in summer. The kittens were cuddly with the dog before. We have recently got the dog shaved and the kittens are terrified of the dog since then and I can't seem to calm them around him. The kittens were perfectly getting along with the dog before he got shaved... Is there anything we can do to help them get along?
This is most likely non-recognition aggression.
Cats recognize other animals by scent, not sight. This means that if one of the animals leaves the home (such as to a vet or groomer) and brings strange scents home, the cats who remained will not recognize it at first and will treat it as a threat.
The only solution to this (at least after the fact) is to keep them separated until the strange scent wears off. It can take a day or two for cats to groom away a strange scent, but dogs can keep it for weeks since they don’t self-bathe like cats do. You can try bathing the dog yourself, if you’ve done that before, but it won’t help if your cats have never smelled your dog freshly-bathed before because that’s just a different unusual scent.
In the future, any time one of your pets will be leaving home, wipe them down thoroughly with a clean towel to collect their natural scent, and then wipe it back onto them when they return. It won’t 100% solve the problem, but it should speed things up.