Hi and thanks for using Petco Pet Education Center, formerly Petcoach! A small amount of cooked beef should be okay as long as it isn't too fatty. The main source of nutrition for your kittens should be kitten food, but a small amount of beef is unlikely to cause any harm. If you note any vomiting or diarrhea after offering beef then I wouldn't offer any in the future. Cute kittens! Good luck with them!