Well, I wouldn't take the hen away from them - I can't think of a reason to do that artificially.
If they've got a mum looking out for them I should think you could let them out now. Personally I'd do it while I was around and watch for too much interest turning into nastiness from the flock. If they're fine, good, if not maybe pen mum and chicks where the flock can see them but not get at them, for a while longer.