hi and welcome to our forum. Margy was correct in telling you to change the diet to a good brand of preferably 'grain free' food. I will give you a site that lists the best ones on the market, however they are more expensive. if you must buy grocery store brands try to stick to CANNED foods that have meat listed as the first ingredient...not meat by-products. canned food is always best for a cat rather than dry.
just remember when changing foods to do so slowly or the diarrhea could become worse..any change in food needs to be slowly introduced.
I would also follow thru with a visit to the Vet, your kitties may well have worms and need to have a stool culture done to rule this out or they will not thrive with any food.
good luck, keep us posted♥
http://catinfo.org/?link=cannedfoods#Commercial_Foods
What food would you recommend? Or just wait to go to the vet.
It may be a good idea to change it is they have diareha and I would certainly take them to the vet to have a check up .it is possibly the food ,