The Children of Hamelin are the characters of the classic fairy tale The Pied Piper. When the mayor and people of Hamelin show themselves to be thankless by giving to the Pied Piper only a single gold coin instead of the promised bag of gold (claiming that the rats left of their own accord while the Piper played on his instrument), The Piper is incensed by their betrayal and decides that their children deserved better than to be allowed to grow up like the corrupt townspeople. So he uses his music to lead all the village children out of Hamelin and into an enchanted world, where they can play and laugh all day long.
The Children of Hamelin appear in Once Upon a Time, as inhabitants of Fairytale Land. Alerted by the disapearence of his son Baelfire, Rumplestiltskin investigate into Hamelin and find that all the children were also missing attracted by a mysterious Pied Piper. This last was, in reality, Peter Pan, corrupting young children before taking them in Neverland, where they serve him like the Lost Boys.