If they are going to leave the playroom as a disastrous mess... then they don't get to play there. They stay with you and the "babies". Playing unsupervised is a privilege, not a right!
They get one last warning about your expectation, and after that, I would go check in on them. If it's getting crazy in there, they come down with you and have to make do with baby toys.
I am guessing they will catch on pretty quick!