The Last Child of Hamelin (Spencer Hill, 2014) by Ray Ballantyne follows up on what happened after the Pied Piper took all the children.
It's 60 years later and twelve-year-old Pieter, sometimes just has to escape from Hamelin. Escape from his abusive father, from the other boys who pick on him, and from where he's forbidden to express the music that wells up inside of him. Up on the mountain he meets the old man Simon--the only child who didn't follow the piper--and Pieter hears the music of the piper himself.
I love how much the author has made me care about Pieter. I think you'll find yourself caring for him too in this intriguing fantasy.