I like to believe that music box melody comes from a music box from the toy box.
Not a huge fan of the melody personally as it's a bit too happy for my liking, but it does fit with the theme perfectly. On the other hand that more intense part starting at 2:26 was fantastic, I'm associating it to the boy's father as it has a similar melody (or is it just the scale? not sure :P) to a view to death.