(Sorry I have been on a Beauty and the Beast kick recently)
Alright so (A) is the child of the town oddball, and just for the sake of convenience let's just say that they ran away from home because they kept getting harassed by other townspeople. They escape into the woods and..it got dark, really fast. After a bit more walking (or running out of paranoia) , they come upon a building (it can be a castle or just a normal, big house if you want) and decide to take shelter there since it looked to be abandoned.
Long story short they got caught by (B) and are now forced to stay there for 'forever', how it plays out from there is up to you.