6 year olds?
Large Trebuchet. In fact a few so the all the neighborhood kids can play. It teaches eye to hand coordination, thinking on the fly, algebra and ballistics, plotting, mindful breathing. When they run out of projectiles they can sit in the sling, launch each other into the next block and learn basic flight characteristics as well as exercises in gravity.
Hugh Latimer Dryden would agree. He graduated college at 14 years of age and won an award. Earned his MS in Physics by 18 years of age and wrote his thesis. "Airplanes: An Introduction to the Physical Principles Embodied in their Use." He kept a model Trebuchet on his desk.
Its safe as it does not involve fire.
