Inspired by Bobby Dunbar

A Pureblooded kid goes missing and is "found" months later... But it's a different kid.

Possible complications:
The missing kid is thought to be (either is or isn't doesn't matter) a squib and intentionally lost and the found kid is a muggle born with proven magical talent taken from his home and "adopted" into a better life.

Another bonus either - the missing kid turns up, is swapped, actually has magic, or the original mother goes looking for her son and knows about magic (maybe she's a witch living as a muggle) and has to try and prove they've taken her son.