I love the stories on this thread.
I have two stories that come to mind, but I don't think either of them made the kids very happy.
I was at a cosplaying as Tsuna from Hitman Reborn at a picnic, and I heard a kid saying something about me being an alien...lol?
When my brother was cosplaying as Hibird, some really young kid's mother said "look a giant duck!" and the kid saw him and started crying.