My kitten has diarrhea and won't eat vet's GI diet. What should I do?

Congrats on your new kitten! I have a Persian as well, and they are lovely cats. Royal Canin has a brand of wet kitten food, and you can try that to see if doesn't give her diarrhea and satisfies her desire to have wet food. You can mix that wet food with her dry food to see if she eats the dry food better. I hope this helps!

Withhold food for so long in such a small kitten is not recommended. Diarrhea is a signs referring to a intestinal problem. It could be due to parasitic infestation (very common in kittens) dietary indiscretion, inflammatory disease, bacterial diseases or viral diseases. You could provide bland food (boiled breast chicken or white fish or gastrointestinal commercial diet) for a 24-36 but, if she continues to have diarrhea, might get dehydrated and therefore need to be checked by a veterinarian. Also if you can spot blood in the faces or if she get lethargic and not eating, or if she start vomiting, I would suggest to have her checked by your veterinarian as soon as possible. 
Good luck and post again on the forum if you need further advices. I hope my answer was helpful.

Diarrhea and a decrease in appetite could be a sign of a number of different problems. Intestinal parasites are possible, even for indoor cats, as are some infections or diseases like colitis or inflammatory bowel disease. If the diarrhea continues for more than 24 hours or seems to be getting worse, or if her appetite does not improve during that time I'd recommend seeing your veterinarian. They may treat her symptoms or recommend some testing like blood work or a fecal analysis depending on what they find during a physical examination.

You can try home made diet and probiotics for a week. GI distress from new food or treats generally only lasts a few days. If the diarrhea persists, worsens, or he becomes uninterested in food, I'd recommend taking him to see his vet for a physical exam.

At 8 weeks your kitten can be on a completely solid diet, and if you're feeding regular cow's milk it could be what is causing the diarrhea (cats can't handle milk.) If she doesn't drink a lot and doesn't like moistened food, you can try adding in a pet fountain or other running water source that may encourage her to play or drink a bit more. If she is still having loose stools even after feeding only her regular food, a vet checkup to rule out parasites and GI upset is also a good idea. If she is trying to nurse your other cat, just separating her for a bit can help deter the behavior.

Make sure he is getting plenty of fluids. I would syringe him water and feed him wet food to get more moisture in him. I would feed a diet of chicken and rice and add in a probiotic such as Purina fortiflora which can be found online quite easily or at your local pet store. If there is no improvement or he worsens, I would recommend having a veterinarian examine him so that the most appropriate diagnostic and treatment recommendations can be made. I hope this helps guide you in the right direction. Please let me know if you have any other questions or concerns and I would be more than happy to help you further.

She's adorable! I would be a little more concerned that she might have intestinal parasites or another disease causing her diarrhea. I would recommend seeing a veterinarian if you have not taken her in already. They can check a stool sample and she may be old enough for vaccines at this point. If there does not seem to be a medical issue you could try seeing how she does on just wet or just dry food - kittens sometimes do better on one vs. the other.