I had a cat with constipation problems, and there was a Science Diet
food his vet recommended. It wasn't bad, but in the end, I solved the
problem by adding canned pumpkin to food. You have to start out a
little at a time, but by the time he was used to it, he would eat food
that was half food/half pumpkin. I used a wire whisk to mix it up well.

Also, if they'll eat it, baby food peas (and squash) are good. Mine
loved the peas.