Welcome to YT, first of all!
At that age, I'd be worried right away - as puppies can get into danger after day 1 with diarrhea. It could be coccidia, giardia or other things. I would get them to the vet asap for fecal tests. Even though they're still active - what can happen with puppies is that they're fine, and then *boom*, they're not. Can you get a fecal sample to a vet right away tomorrow morning?
I would also recommend BeneBac from a petstore immeditately, you could also give canned white mashed potatoes tonight (take the food away).