Can a single player block another player below the knee in American youth football?

A block below the waist is called a "cut block," and the rules regarding this can vary among different youth leagues, so it would be best to consult a rulebook or a league official.

At the high school level, cut blocking is allowed within the "free-blocking zone," defined as a rectangular area extending 4 yards laterally from the ball and 3 yards to either side of the line of scrimmage. Once the ball carrier has left the zone, the zone no longer exists.

A chop block, however, is never legal. This is when a player gets cut while already engaged by someone who is blocking high (above the knee).