Better the kids run around the departure lounge than go bonkers onboard. We've always managed to find an empty area of the lounge, like a very far gate or seating area and let him have a dash around there, obviously if a flight needs that area we move. It's not like I'm suggesting going down to the Biz lounge and having a child go mad there.
Normal people understand. Travelling with kids isn't easy, but if you prepare, like the OP has obviously tried to do then isn't that better for all. Please do not suggest not travelling with Children, if we all kept our Children at home and never went anywhere how would we raise a generation aware of others and their cultures and Countries?