How many kids do NOT have it yet? It is usually most contagious before the blisters. So if 3 kids already have the blisters then odds are everyone has been exposed to it for a good week now. I would be tempted to tell the parents of the kid(s) that have NOT showed symptoms that 3 kids have blisters, meaning everyone has been exposed for over a week now and it is up to them if they wish to keep their child home for a couple days to prevent exposure while you clean the place and the others recover.

It just kind of seems pointless to send ALL the sick kids home when they are less likely to spread anything...especiall y since your home is likely also contaminated!

Then, the parent of the healthy child can decide if they want to take the extra precaution by keeping their child home (where it is not contaminated)!

NOTE: this is just my thoughts and I don't really have first hand knowledge if this is the best plan!