Thank you for fostering these little sweeties
Good choice to buy some canned for them
. Cats need their moisture with food especially when they have diarrhea.
You cats don't need rice, they are carnivores, they need meat. You are much better off giving them raw meat. Unlike humans, cats get their energy from muscle meat and animal fat. Here is a good link on cat nutrition:
You should see a difference within three days.
Good luck and if you have time, sure would love to see some pics of them