I would let them out when they're about 4 weeks. I had one hen sitting on eggs twice with only about a month or two in between. We moved her and her eggs into a little house with a run attached, fully secured, but inside the larger run that the other chickens have (both times). The first lot of 8 chicks, we let them out with the hen at about 2 months, which I think was far too late. Now, they're about 6 months and they don't socialise with the other hens, they don't roost in the hen house (they roost in an open shed close to the isolated house/run they shared with mum), don't feed with the other hens+rooster, seem to have created their own separate pecking order (there are only two left now - 6 were roosters, had to cull unfortunately), and they generally just seem a little naive. By the time we let them out, they were big enough to fend for themselves - yes, but mum realised this, and pretty much left them to their own devices, so they didn't know how to be proper chickens, in a sense. They developed an unhealthy attachment to the run they were raised in also. Also, I think mum started getting stressed and frustrated being in such a confined space.
The second lot of 4 chicks that the same hen hatched, we let them out after 4 weeks. They're about 2 months old now, and they roost in the hen house, they eat with the other hens, and generally seem a lot more adjusted to being a part of the chicken family, as it were.
So there's my reasoning - I'd say about 4 weeks . And I've found that as much as I worry about other hens, mum will look after her chicks. What I did though, for safety, for the first week that they were out I only let them out of their separate run during the day, and I kept an eye on them to make sure mum was doing her job. As long as the chicks have been in visual contact with the other girls, and the other hens are used to them, it should be okay.
Good luck!
