Tonton Streaming My Three Sons (1960) S1E24 - The Lostling

Chip begins to think it would be great to be an older brother, so he wishes for a little sister. After the new Hawkins family move into the vacant house across the street, a wild sequence of events results from an improbable case of mistaken identity -- an infant is somehow confused with a leg of lamb left in the boot of Steve's station wagon.