OT:
MK, mother cats always clean the bottoms of their kittens in order to get them to void. Kittens (and puppies) bathroom functions are not fully developed when they are born, so mommy cat/dog helps to stimulate that function by cleaning them. Since your little Cheeto has been sick, mommy cat may not be giving her the attention that she gives the other kittens. You stated that you got a LOT of pee when trying the warm water trick. This indicates to me that mommy cat is not doing this for her. Neglecting that bodily function can be very dangerous for the kitten. My opinion, as a long time owner of dozens of cats/kittens over the years, is that you are going to have to probably take the place of mommy cat in performing the "going potty" ritual. When I was having to care for kittens in a similar situation as Cheeto, I used a warm wet wipe or washcloth on the kitten's bottom after every feeding. It takes several weeks before the kittens are able to use the potty on their own without the assistance of mommy cat's cleaning ritual.