well, you may consider this drastic, but they must get used to eating veggies, so try not to give them food for a half a day or so, and then give them nice boiled chicken with carrots maybe, or something...I've read that you can cook almost everything exept onion and garlic...so here you can read the list of bad foods for dogs(since i don't have a cat, you have to look it up)
http://mooreshaven.com/pets/dogs/saf...foodslist.html
and remember no spices and no salt!And use supplements