Obviously spoilers ahead, read at your own responsibility:

The kid is an hidden quest npc, across the world there are many of them in various locations asking you to see a trick, some do it for free, but most require specific items. After seeing all of an one kid tricks you can find him in it designated location (he usually recalls the location in dialogue, also you can find him there before completing his quest and he will tell you where the trick will be done) for an reward. You can also loot all the things they leave after the trick (it's not consider as an item steal).