Skull kids are children who wander off and get lost in the forest and then adapt to the enviroment, i also believe that children that are tired of civilization or just tired of their friends or/and family wander off in search of the skull kids, wishing to be one of them.
i also believe that just because the
tp one knows sarias song doesnt mean that he's the Skull kid that appears in MM, there's a lot of skull kids, and i think they're aware of each other, i think they conduct celebrations and rituals, speaking and playing with each other and helping other skull kids in need. however i do think that even though they have this sense of companionship they like to spend most of their time alone. i think that's why they've passed down Sarias song, to remind themselves of the place they originally came from.
i also think that they control the woods in some way, can create illusions to play with and trick people that are walking in the forest, making them wander lost forever.
also, i believe that the skull kids never die of age, that they never grow up (much like kokiri) and that they are eternal beings watching over the forests, and that every single one of them has a different story about how they turned into the creatures they are today.