is Child Ballad
The title knight tries to seduce the queen. When she rejects him, he puts a leper in her bed and told the king that she was unfaithful — and not even with a right good knight. The king orders them both executed. The queen demands a trial by combat, if she can get a champion. A messenger in search of one is sent back by what looks like a four-year-old boy, but who knows what the queen dreamed. He appears and mortally wounds Aldingar in the fight. Aldingar confesses. The king and queen are reconciled, and the leper is miracluously cured and serves the king as a steward.
Francis Child's variants listed here