Children of the Moon

Children of the Moon
The Children of the Moon are another of the mysterious aspects surrounding the strange final dungeon in Majora's Mask. 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 masks that are dropped once the boss enemies are defeated while the fifth wears Majora's Mask. The Children are each 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.

The Children's behavior is also odd. The four ask for different amounts of masks from Link. In return, they send him into types of mini-puzzle dungeons that are thematically similar to the dungeon ruled by the boss who's mask they wear. Each of these teleportations is accompanied with a enigmatic, slightly disturbing question that concerns Link's, and the player's, beliefs about the world around them. The children casually refer to the life-threatening dungeons they send Link into as "games". After Link returns from each dungeon, the Child leaves Link alone, their purpose completed. However, if Link did not obtain all of the masks in the game, he cannot access all of the puzzle dungeons and, subsequently, cannot get the Fierce Deity Mask.

The final Child, the one wearing Majora's Mask, will send Link into the final boss battle with the Mask itself if asked. His behavior mimics the other Children as he refers to the fight as a game and offers a disturbing question, but he mentions how the other Children won't play with him. Furthermore, if Link successfully 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, the most powerful transformation mask in the game.

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. The Salesman mentions a tribe that once owned Majora's Mask. Some believe the Children are the last remnant of that tribe. 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 won't play with the one wearing Majora's Mask. 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.