Three teams, 1 ball: pass and cross behind the player.
Do this slowly at first, U12 kids find this a difficult exercise. Focus on passing and running. Start together (half field lengthwise) and build up when it goes well.
Depending on the size of the group, divide the field.
Variant for when it goes well:
Finish with score and play 2x1 (the one who scores will defend),
Pay attention to outlet pass (ask for the ball on the sideline).