Are the kittens ill, ie sleepy and inactive, if so take to vet. If they are fine in themselves I would think food intollerance and feed them cooked chicken for couple of days is this is very bland. Then gradually (by mixing small amount in with the cooked chicken) over a couple of days change them onto good quality wet food such as Hi-Life Kitten pouches. I would think the reason they got worse was because of sudden change from dry to wet food ... changes need to be made gradually with kittens.

In my experience (had a persian many years ago) water was the culprit with his tummy upset. As soon as I started giving him boiled and cooled water from the kettle he was fine.

Hope things improve soon