Guess it depends on temperaments. I had five chicks with broody mum in a run where adult hens could wander round and see the family. Think I kept them in for about 4 weeks, then let them out. Kept an eye on them at first, but with a broody mum and plenty of space, I saw no problems. Chicks still sleep in their own little house, but spend all day free range with everyone else.
If it was a choice between having a tiny run turned to mud or messy for them to stand in or being out with your few other hens, I'd let them out.
But then I have lots of space and only feed grains to adults (chicks get crumb in their house).