Trying to think logically. Flu is believed to be spread by sneeze/cough/talk droplets from the sick person (or carrier) landing on someone uninfected. If a child can remain contagious 7 days after starting symptoms, then the oldest would likely be non-contagious by Monday, and the youngest, by Wednesday. IF you decide that they can come back, be sure to have them wash their hands immediately upon entering, so that they aren't touching infected surfaces and passing it along to your household. It isn't unreasonable, IMO, to ask that they not come back until everyone in the household is better yet again.