Kingdom of Youth
From the Super Mario Wiki
The Kingdom of Youth is a land in the The Super Mario Bros. Super Show!. The Kingdom of Youth appears in only one episode, "Two Plumbers and a Baby" and is full of giant letter blocks, and other things babies like. The center of this land has the Fountain of Youth, a machine that can alter anyone's age. Mario, Luigi, Princess Toadstool and Toad came here, only to be attacked by King Goo Goo Ga Ga Koopa and get the princess turned into a baby.