SIL's Kittens - Vomiting & Diarrhea

My SIL got 2 kittens about a month ago for my nieces. They are now about 15 weeks old and have been great. However, in the last 2 days one of the kittens (Sam) had a couple of rounds of diarrhea and then vomiting. Today they other one, Grayson, vomited. They have not changed their food, they've always been fed all natural, human grade kitten food. The kittens have not been around other animals or anything like that. My SIL called the vet but hasn't heard back yet. Does anyone have any ideas as to what this could be or advice on how to take care of the kittens until she hears back from the vet?

I know there is probably a wide range of possibilities here, but any experience with this would be great! TIA!!

ETA: Both cats came from a neighbor's cat and have been well taken care of since day 1. Also, they've both been to the vet and have been de-wormed, so I don't think that could be a possibility.

Just wanted to update, the kittens went to the vet yesterday and were tested for all of the most common ailments for upset stomach. They were negative on all of those tests. The vet mentioned that it could be teething and also gave them a general anitbiotic to cover several possibilities. Luckily they were not dehydrated, so now I guess it's a waiting game to see if the antibiotics work.