Moon Child

The Moon Children are characters in.

Biography
The Moon Children are located Inside the Moon. There are five children, four of which are running around the tree in the center of the field, while the fifth sits at the bottom of the tree. The four running children each wear one of the Boss Remains while the fifth wears Majora's Mask. The children are identical in height, clothing, and hair color, differing only in the mask they wear. Their gender is debatable but their haircut seems to indicate that they are male. They were created by Majora's Mask, made to appear the same age as Link. Despite their resemblance to the Happy Mask Salesman, he had never met them.

The four children ask for different amounts of masks from Link. In return, they agree to play Hide-and-Seek with him, sending Link into a dungeon thematically similar to the one ruled by the boss whose mask they wear. After completing each dungeon, the child at the end of it will as Link a question that concerns his beliefs about the world around them, after which the children will ask for more masks. After Link returns from each dungeon, the child disappears. However, if Link did not obtain all of the masks in the game, he cannot complete all of the dungeons and thus cannot get the Fierce Deity's Mask.

The final child wearing Majora's Mask will send Link into the final boss battle with the mask itself. He does not ask a question, but he mentions the absence of the other children if Link has completed the four dungeons. If Link obtained every mask in the game and gave them to the other children, the final one will comment on this and give Link the Fierce Deity's Mask. Before the final battle stars, he refers to it as the good guy playing against the bad guy.

Theories
The children's only appearance is on the Moon. They are not mentioned or seen in any shape or form in the rest of the game or the rest of the series. The only ideas regarding their origins are mentioned in an offhand comment while the Happy Mask Salesman describes Majora's Mask early in the game.

Tribe Members
The Salesman mentions a tribe that once owned Majora's Mask. Some believe the children are the last remnant of that tribe.

War Victims
Others believe Majora once waged war on the tribes represented by the boss enemies and the Fierce Deity. The children could be the spiritual remnants of those tribes, which would explain why the other children will not play with the one wearing Majora's Mask.

Happy Mask Salesmen
The children look eerily similar to the Happy Mask Salesman (haircut, pale skin), and even ask Link if he will also be a Mask Salesman.

Memory of the Giants
One theory proposes that the children are the memories of Skull Kid's friends before they became the Four Giants. After they became gods, Skull Kid used the power of Majora's Mask to imprison them within their own masks and transformed the masks into bosses, and the child wearing Majora's Mask would be his idea of himself now and how the giants would not interact with him.

Trivia

 * Hacking into files reveals that the face models used for the children is that of the Happy Mask Salesman. In the original game, their faces had no texture.