I let the chicks decide when to roost, as they're inclined to follow the adults' example. But part of the reason that works is because the brooder gets closed up as well.EE is 6 weeks old now and BO is 3 weeks old.
I attempted to get the chicks and put them up on the roost at night with the other hens once they settled down for the night. After some serious crying by the roo, all calmed down and it looked like they would stay there comfortably. I put them next to one of my nice girls who does not peck or chase them.
This however, did not work. I went outside a few minutes later only to find that the chicks had hopped off and had gone into their brooder area. Bummer.
Of course the brooder area is going away at some point, but with a long weekend trip coming up soon (2 weeks or so) and the girls needing to get through a lot of the layer feed, I figure I can have the chicks in there a couple more weeks. Once that trip ends though, I think I'm taking away the brooder area for sure.
The 3 week old probably still needs heat so for the time being I'd continue to let them have brooder access. You can probably close it up next week if temperatures hold steady, so that gives you 1 week to try and get them to transition to roosting.