Well we are lucky enough to have a playroom, it is downstairs, but DDs and DSD (6,4,4) go in, shut the door and we don't hear from them for hours sometimes.
They love playing together on their own IYSWIM.
If you think DC is too young to be alone in his bedroom can you get your DC to tidy the toys? DD1 has just turned 6, and she tidies properly and has done for quite a while now. DD2 and DSD help fairly well too.