Take the kitten to a qualified vet. Get a specific diagnosis. do they have a parasite, sickness, poor diet, or many other symptoms. try some simple things at home, change their diet. try changing their routine at home, first.
First consult a vet. Make the kitten to sleep on a soft mat in a seperate place. Change the mat every half an hour. Do not feed the kitten in the night time. Feed only milk to avoid dehydration. Mix electrolite with the milk to arrest diarrhea. Let recover soon.
A kitten with diarrhea, especially in its sleep, could be showing crucial signs of dehydration. Make sure the kitten is ingesting fluids regularly and also make sure that you keep track of the consistency of the stool.