One child is the leader, the other is the shadow. Both have a ball. Leader must try to lose their shadow and the shadow must stay with their leader. Set a time limit eg 30 secs or 1 min. When the time is up, see how close the shadow is to its leader.
Variations: Introduce gates that or obstacles. If the leader goes through a gate, the shadow must follow.
Tell them they must work together to be near each other at the end.