Did they receive vaccinations? My vet told me this year that they've added a new vaccination for a strain of leukemia (I think it was leukemia) to the shots and it can cause especially kittens, but I've noticed it in Aroara also, to be lethargic and uninterested in food for a day or so. Perhaps that is the situation with yours. If it hasn't improved soon, I'd be calling the vet's office.