I have gotten hens to even sit on older chicks. The way I would do that is at night put the chicks under her and let her sit. (nest box door covered) then I would take them away in the day, I would let them sleep with her 3 nights in a row, then put them together in a brooding area. The above chicks are probably 3-4 weeks old in this pic, and had been with the mom for 2 days.