You say they have plenty of room to roost, could you pleased translate that into feet and how they are set up? One long roost or two or more next to each other? Maybe a ladder roost? A photo would be great. Are the only chickens you have the hen and her chicks or are there other chickens in there?
It’s not all that unusual for chickens to be kind of rough when they are settling in for the night. The ones higher up in the pecking order can be pretty brutal when they kick the weaker members out of the preferred roosting spots. Especially when you have immature chickens a bully might go out of her way to pick on them. Without knowing a little more about your specific situation it sounds like the ones lowest in the pecking order are being picked on when they go to the roosts so they are looking for safer places to sleep. That’s often a nest box.
I’m always integrating younger chickens into my flock. This “roost behavior” is so common I put up a juvenile roost, higher than the nests but a little lower than the main roosts and separated horizontally a few feet. It made my life easier.