The Twins' Room
From the Super Mario Wiki
The Twins' Room is small in size and filled with childrens' toys, such as trucks on the shelf, model planes hanging from the ceiling, and a globe on a desk near the door. A pair of tennis rackets is in the corner next to a desk cluttered with books and a poster of the "Monsters" is on the wall. A Speedy Spirit is hiding in the bunk beds.
Ghosts in The Twins' Room during the blackout