The Shoemaker's Boy by Joan Aiken

From School Library Journal:
Jem, the shoemaker's son, is left in charge while his father makes a pilgrimage to the Holy City of Saint James to pray for his sick wife's recovery. In his absence, an extraordinary sequence of events transpires, involving three green children, a black knight, a white knight bearing three keys on his shield, and a mysterious package. After Jem survives the challenges of one terrifying night, a miracle occurs-his mother awakens desiring exactly what the package contains.