Go to the Mushroom Kingdom. Find the home with the kid jumping on his bed, on the 2nd floor. Talk to him and encourage him and he'll jump faster. Now, jump on his head and jump up and down on top of it. Sometimes you'll slip through his head even if you don't move the Control Stick. If this is the case, you'll be stuck inside of the kid and you can't move. To get out, jump and you'll get out of the kid.
Submitted by: kornyhatemachine