Fan Casting Hidden Backup Prince in Sort by Tropes

You have a setting with a monarchy, or where heredity is somehow important and the entire royal / Super Family Team are under constant threat of being killed in a surprise attack by their enemies. The solution is to (intentionally or unintentionally) raise one of the King's children as a commoner far away, without knowledge of their Secret Legacy. The idea being that they'll be safe in anonymity, and they may even be able to be trained or schooled in a way that spares them from becoming snobby and brings them closer to commoners.