It sounds like your kittens have an upper respiratory problem, among other possible things. If you are seeing green snot or drainage, you need to see the vet for antibiotics. I would also suggest that you deworm them, since you are also seeing diarrhea.
The good thing about litters of kittens is that most vets will see the whole litter and only charge for one office visit. If money is a concern, be sure and ask about this before you go.
Let me know if you still have questions.