Play the "Childhood Memory" level. Once in the mirror universe, you will find two dollhouses sitting beside each other in front of you. The dollhouse on the left has stairs. Climb the stairs to reach the third floor. Look towards the other dollhouse. Make a running jump across to the other dollhouse's third floor (on the PC version, press [Shift] + W to run, then [Space] to jump). If a new scene does not start, turn around and move towards the wardrobe near the bed.