Should I be worried if one of my kittens is sneezing? She doesn't sneeze all the time, and apart from this looks pretty healthy although she's the smallest (now 190g born thursday night/friday morning)
I wouldn't worry just yet, but if the sneezing is accompanied by thick, yellow or green mucous, then isolate Baby from other kitties in the house and get her to the vet.
Considering that your kitties are not a part of a responsible breeding program, why not go ahead and get them spayed and neutered?
I think every second post I have had to make on this forum is to confirm that their mum will be getting spayed soon. As for the kittens I think 6 days is a bit young!