here's a good article that explains kidney disease very well. No where does it say that kidney problems are CAUSED by high protein diets. (Although once it is diagnosed, you go to a lower protein food).
Kidney Disease: Causes, Signs, Diagnosis and Treatment in Dogs and Cats
Any way they could have gotten into antifreeze? It smells good to them and tastes sweet. Just a few licks can cause kidney failure. I still think it could be some sort of toxin or even a hereditary quirk rather than treats.
Good luck.