She probably realises that if she roosts the wee one can't get up with her yet - can you make a wee ramp (rather than a ladder) that the wee one can get up to the perch?
We have small (but not bantam) marsh daisy that keeps her two with her in a guinea pig run. She free ranges with them during the day but goes back in the gp run at night as there's not perch, her two chicks are getting quite big now not babies.
For a long time we had free range hens that used to roost in the trees - they would stay on the ground with the chicks until we put up a ramp, then the babies learnt to climb the ramp with their mums and their mums would sit on a tree branch with her wings spread over her young.
I wouldn't worry about the cold yet, they'll be fine for a month or so and chicky will be bigger by then.