O dear, I'm afraid you've got something very nasty there. I went through a stage where I was losing whole litters like that and we never did get to the bottom of it. It was several years after that before I began to relax with kittens and diahorrea! The trouble is at 6-7 weeks they are so very vulnerable. I do hope they will be OK but if not, please put everything on hold if you can, and if you have any other litters on the way or below 6 weeks disfinect, isolate, use kitten pens, outdoor pens, whatever you have to. Eventually you will clear it and then you can get back to normal.

Liz