NET
43 But when the feast was over, as they were returning home, the boy Jesus stayed behind in Jerusalem. His parents did not know it,
Darby
43 and had completed the days, as they returned, the boy Jesus remained behind in Jerusalem, and his parents knew not [of it];
KJV
43 And when they had fulfilled the days, as they returned, the child Jesus tarried behind in Jerusalem; and Joseph and his mother knew not [of it].