tooth fairy tradition whereby children who lose their baby teeth (milk teeth) hide them under the pillow at night for the tooth fairy to find and take away in return for a small sum of money. According to the legend, the fairy uses the teeth she gathers in the building of a new castle.
The mention of it here is appropriate as the story is partly a rite of passage tale. Fitting too that the boy's sister combines one tradition with another when she examines the gap in his teeth and asks, to his embarrassment, "Who's been kissing the girls?" This is a reference to a Northern England legend/joke that kissing the girls causes you to lose your baby teeth.