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This is a PERFECT way to introduce D&D to a party of newbies.
Maybe make the child mute (so that they cannot say their own name, escaping the oldest trick in the book) or simply make them very nice to the fairies and respectful of the nature thanks to, idk, a grandma Druid.
If you take the grandma Druid you can make that since she was very rich thanks to her adventures, some bandits kidnapped the child to have for ransom. Then the Grandma, knowing the relationship with the fairies that his grandson had, asked for their help.
For 10+ years a child goes to a forest to play/talk with the fairies there despite local legends of their sadism/cruelty, and afflicting fates worse than death. So when the now 16 y/o child is abducted, the traffickers find out not all local legends are BS when the fairies come for their “pet”.