I think you have a valid concern, especially since the young ones are Silkies. They can't fly so roosting can be problematic.
First though, I suggest opening the nests now. Chickens can start laying as early as 16 weeks. That is really unusual but it can happen. 18 to 20 weeks or even later is a lot more normal for the first egg from a flock. They are creatures of habit. You want that first egg to be laid in the nest, not somewhere else.
I would want to know if you have a problem before they start to lay so you can work on fixing it before you get eggs with poop on them. Also, the chickens will almost certainly play in the nests before they start to lay. If they scratch the bedding out, they are telling you that you need to raise the lip on the front of the nest.
It is possible those young Silkies will try to sleep in the nest boxes to avoid the older birds. I'd guess at that age they are still sleeping on the floor and not trying to roost. Not all Silkes try to roost but are sometimes content to sleep on the floor, while others will want to get up high. Older chickens can be pretty mean and brutal to younger birds on the roosts as they are settling in at night. I've had regular breeds of young birds leave the roosts because of that brutality. They can start sleeping in the nest boxes even if the roosts are higher than the nest boxes and even if they had been used to roosting on the roosts. I've had chicks raised by broodies do exactly that. They are used to sleeping on the roosts, but when the broody weans them, they get picked on so much they leave the roosts and look for a safer place to sleep.
What I suggest if you have room is to build a separate roost, lower than the regular roosts and separated from them a bit. Make them higher than the nests but not high enough to tempt the older chickens to switch to them. Then give the silkies a way to get to them. such as a ladder or just a pole on an angle they can walk up.
Good luck!!!