I'd also have the mother checked. There's a possibility that it's feline leukimia; the symptoms you describe and the death of a whole litter...there's a strong possibility.
Well it is very common to have kittens die of dehydration. You mentioned they had diarrhea. Diarrhea may be caused by parasites. If mom had parasites before giving birth she passed it on to her babies. I had a german shepherd who had a litter of 8. All of them died. Cause? Parasites transmitted through the placenta.
Also, if one of the kittens got URI it could have passed it on to the rest of the litter. If the mom isn't healthy, the kittens will most likely be unhealthy, and being the fact that their tiny bodies can't handle illnesses, they consequently die.