Hi there. I breed pedigreed cats for about a decade. So, I raised a lot of kittens in my home. Akaisa is right, first you need a safe room.
I think this might be coccocidia. It is a parasite. Most cats have it, but when the immune system is depressed - which often happens in transition - there is a flare up. Kittens are especially vunerable, and can die from dehydration pretty quickly. Even if they normally eat dry food, I would feed them wet until they recover.
Your vet can test for it and give you drugs for it. The drugs interupt the life cycle of the parasite, which otherwise continues living in the intestinal wall. Aside from the fact that a smaller space is less stressful for kittens, it is imperative to keep them clean and away from their feces to avoid reinfection. Even a bathroom. As there are two, they will have company. And you can keep that space clean until this clears up. When it does, just open the door. Leave the boxes and food inside. They will use that a a safe home base to acclimate to the rest of their enviroment.
I only had it once in ten years. And it was awful. I put the whole litter in the bathroom. So much easier to clean. They all got healthy in a week or so.