Really. It is not four children, a tree fort, a couple of climbing harnesses, some ropes and a lot of only-marginally safe crazed swinging, leaping and 'flying'. It is not an accident waiting to happen. It is not inadequate parental limit-setting.
No, it's pulleys, pendulums, mass, density, period, acceleration, distance, speed and moment of inertia.
It's physics. Really.