Respiration/circulation: Choose 2 kids for leg muscles, 2 for lungs, 2 for heart, 1 for the brain, + other organs as you wish. Arrange them around the room. 2 are the red blood cells travelling around the body. Lungs have a store of "oxygen" (poly balls or other unbreakable objects) which they supply to the blood as it passes by. Blood then needs pumping, so visits heart. Then to muscles, brain etc. where it hands over oxygen. Then needs pumping again, so back to heart. Then needs oxygen, so off to lungs to replenish supplies. Hopefully the brain can give instructions once you've got it going.