Lunar Children

The Lunar Children are characters from The Legend of Zelda: Majora's Mask. They are five children living on the Moon. When Link arrives on the surface of the Moon, he finds a grassy plain with a tree in the center. The four Lunar children wearing the masks of Odolwa, Goht, Gyorg and Twinmold can be found running around, playing in the grass, while a single Lunar Child wearing Majora's Mask can be found sitting under the tree.

Each Lunar Child has their own dungeon that Link must complete in order to receive the Fierce Deity's Mask. In order to enter the Odolwa child's dungeon Link must surrender one mask to enter, and one mask to leave. The Goht child's dungeon requires two to enter, and two to leave. The Gyorg child's dungeon costs three masks to enter, and three to leave. Finally, the Twinmold child's dungeon requires four masks to enter, and four to leave. When Link finally completes all these dungeons, and gives up all twenty masks, he will be given the Fierce Deity's Mask in return. The Fierce Deity's Mask is not required to beat Majora, but it makes the final boss battle much easier.

When Link talks to the boy sitting under the tree, he will ask if Link wants to play a game of "good guys against bad guys". If Link accepts, he is entered into the final boss battle with Majora.

Theory
The Lunar Children all bear striking resemblances to younger versions of the Happy Mask Salesman. This combined with the fact that they all seem to be infatuated with mask selling leads to the possibility that they are somehow connected to the Happy Mask Salesman. The Mask Salesman may even be a former Lunar Child, who is originally from the Moon.