The Moon Children are allies of BEN and the HMS (Happy Mask Salesman). They appear in Jadusable's dream in the story.
The look like kid versions of the HMS,but wearing masks to cover up their hideous faces, eyeless, bloody, things that would scar you for life.
They travel in a group, there are only 4 of them. Each one wears a mask of each boss you defeat throughout the game.
They are called the Moon Children because they are found in Majora's Moon when you're about to finish the game. If you play with each of them and give away all your masks, you'll get the Fierce Deity Mask. The strongest one in the whole game.
In Jadusables case, they torment him as well as BEN and the HMS. They may be small, but have terrfying power. They don't come in right away in Jad's story, but, towards the middle or end. I also think in the notes.