Im new too!
They do roost, but the perch has to be very low ( only 3-4 inches high, to get them off the ground and more comftable)
They cannot get high like regular fowl because their feathers break up(which creates the 'silkie' appearance)
This means that their flight feathers cannot catch the air to push upwards, and so cannot fly.
They are also very small. If I were you, I would put another perch down lower, but higher than the nest box, even if you need to equip a ramp. They prefer not to roost with bigger birds, but will if its necessary. And so if it is a high perch equip a safe ramp, and teach them to use it using corn.
At the mo i have 7 silkies.
I'm so glad you asked that question -- I have two little silkies, and noticed they can't fly as well as the other chicks . . . but assumed it would get better when they get older. Now I know better! Welcome!