IOTL, King Henry I of Castile, the youngest surviving son of Alfonso VIII and Eleanor of England, died in 1217 when he was struck by a falling roof tile. His death led to the succession of his sister Berengaria who then abdicated in favour of her son, Ferdinand, the heir of Leon. Then, when Alfonso IX died in 1230, the two crowns were united for the first time since 1157.
Aside from the obvious continued separation of the crowns, how would history be different if the boy king had survived? Henry was only 13 at his death. First, I imagine he might marry Elisabeth of Swabia instead of Ferdinand. A delayed Reconquista?