My Three Sons (1960)

S1E24 The Lostling

Chip starts to imagine how wonderful it would be to have a younger sister, so he makes a wish for one. When the new Hawkins family moves into the empty house across the street, a chaotic series of events unfolds due to an unexpected mix-up—an infant is hilariously mistaken for a leg of lamb that was inadvertently left in the trunk of Steve's station wagon.