I built a small 4 x 4 x 4 ft cube with one side a door & hardware cloth, facing the 2 other full size coops.
4 broody hens over 2 seasons have raised their chicks in there, until they decided on their own to roost with the other birds. They are let out with loose supervision early on, and return on their own at dusk. They sometimes will go back in midday, for a rest from the other birds. They may spend several days locked it, and happy to get out, but back they go. I try to discourage the other birds from poking around looking for treats, but they seem to resolve those territorial disputes somewhat quietly. (Black Marans)
In each case, the chicks had already jumped a foot & 1/2 out of the nest box, so I find the pair of them in a corner in the coop. Moving the chicks first encourages Mom to follow. Within several days, and plenty of sprouted white millet treats, the nursery becomes home for 3-4 weeks, at least.