I've gotten creative over the years when it comes to trying to get chicks to roost.
Why don't you try fixing up a temporary or permanent perch for the chicks that is just six inches high? This will give them their own place to roost as well as being something close to the ground and not to be feared. Rig up two, on second thought. The big girls will want to try the new low perch, too.
Or you can try placing the chicks on the perch after dark. When I do this for a few nights, they often get the message.
If the chicks do begin roosting on the perches, and the big girls knock them off, try rigging partitions along the perch. I find this useful even after they grow up.
Closing off the nest boxes at night is a definite must. It's something I need to do as a matter of routine, even though all the flock should know better by now. There's just something cozy and irresistible about a nest that they all want to sleep in them at some point.