I call them playing on the roosts during the day perching. They like to play on high things. 3" to 4" off the top of the coop floor isn't much but it should be noticeable to them. Roosting is when they sleep on the roosts. You are correct to separate perching from roosting, two different things.
My brooder raised chicks don't all start roosting at a certain age. I've had some broods start as early as 5-1/2 weeks. Most of mine start around 10 to 12 weeks, but I have had some go several weeks longer. Having adults around changes this timeline dramatically but you don't have any adults. I think how the coop and roosts are set up influences how early they roost. That includes height, access to them, even where your windows are. We are all unique. If they are Silkies, some never roost and enjoy sleeping on the floor. There could be a lot of different factors.
I personally do not worry about where they sleep or when they start roosting as long as they are not sleeping in nests. If they sleep in nests I have an issue. Otherwise they will work it out in their own time and in their own way.