I could be wrong and it might depend on their age, but I believe famciclovir can be used by kittens, and it can tremendously helpful with feline herpes. It might be worth seeing if the vet would be open to trying it! (One of my little guys had a severe case as a kitten and the rescue used it to bring him through it, but I’m not sure how old he was when they started the course. I adopted him at six months.)