Hi Laurie,
If possible, I would see if you can collect a collective stool sample (from multiple kittens) and see if your veterinarian can send it out. Intestinal parasites is still a first thought. No single dewormer will cover all of the possible parasites young kittens may have, so sometimes that provides a specific cause to target. Although you mentioned you used one, there may still be another one that would be needed.
You could also be dealing with viral diarrhea, for which there is little that can be done other than supportive care, especially hydration. You did not mention how young the kittens are. I’m assuming they’re past nursing/weaning age and getting KMR? If so, you can try adding some water to canned food and dry food to encourage further fluid intake.
I do think a Proviable kit, made by Nutramax (and available without a prescription on Chewy, etc.) may be of some help. The soluble fiber/probiotic paste can really help to at least slow down watery diarrhea in many cases. The dose for all cats is 1ml though if you have very tiny kittens that are smaller than maybe 5lb, 1/2 ml may be adequate.
Canned pumpkin is okay as a mixed fiber source, but it’s hard to predict how much will help or would be needed and you can’t give so much that it’s throwing the rest of their diet off balance.
I hope that helps.