Warm up for Kids can be any game or obstacle course. They can dance, play tag or sing Head Shoulder Knees and Toes as fast as they can. For them, it is about getting them excited for the class, get their blood pumping and their brains firing.
Warm ups for Teens can be more organized. Prepare the kids for whatever movement will be focused on that day. If they will be deadlifting, stretch their hamstrings and have them do some weightless good mornings. Have them run a short distance and perform some wall squats. They can even perform a short bodyweight workout (not for time). Get their heart rates up and their muscles prepared for the workout ahead.