I don't think so. Besides, sharing docs with a group only works if the recipients click the link in their email. Sharing a doc with a group does not put the doc in their docs list by default. So even if there were a way to add them outside the control panel, the end result of sharing docs would still not be possible.
Is there a reason you don't give the kids email but restrict it to the domain only? That's what most of our districts do. -Joe