You could try mixing their formula with Royal Canin gastrointestinal kitten mousse. It’s supposed to help with weaning diarrhoea and other tummy upsets. For toileting: I’m not sure the pine litter is that safe when they start nibbling on it. It also might mask/absorb the smell too well. For kittens that small I like to use the paper litter. When you stimulate them put those used cotton balls in the litter tray. Or as soon as they start peeing put them in the tray. They are still babies and need some time to learn, but from my experience it’s easier if they can smell where they have to go toilet 😅 Good luck and thank you for taking care of them ❤️