My son is 5, Mindstorms are too complicated to investigate but… By hacking a wire straight to a battery, it’s easy to explain to him how to apply power to a motor. No indirection from the programming or the brick, simply energy applied to motors.
With this you can make a simple car:It’s been great fun to watch him figure out how various gears work. Now for the project, we simply turned this car into a robot which climbs up a string.