Children...how young are we talking?
At our club our "wee warriors" (4-6) don't spar at all
Our 7+ do spar, moderate contact, full gear (foot, gloves, mouthguard, headgear) Everyone is watched closely and there are no chances, you intentionally go out of control, you hit to hard (on purpose or after a warning) or your'e not listening and you sit out. time varying of course. The kids know this is no joke and treat it well, rarely are there sit outs.
Past 13, we spar everyone and anyone, obviously the blackbelts and big adults go to your limit, and everyone makes sure that everyone is comfortable yet challanged.