Lone Wolf and Cub: Sword of Vengeance (1972)
Lone Wolf and Cub: Sword of Vengeance (Japanese: 子連れ狼 子を貸し腕貸しつかまつる or Kozure Ōkami: Kowokashi udekashi tsukamatsuru, literally Wolf with Child in Tow: Child and Expertise for Rent) is a 1972 film directed by Kenji Misumi, the first in a series of six Ogami Ittō, a wandering assassin for hire who is accompanied by his young son, Daigoro.
More at Wikipedia.