If you know a kid, then you should also know Tinkercad. Tinkercad is free and browser based with nothing to install.
Tinkercad is the perfect exploration and fun tool to create things easily in 3D and even 3D print them into physical creations. It has been amazing to see what kids (adults too) create by using Tinkercad. It provides an easy way to understand spatial relationships almost like using the modern tinker toys or playdough. You don't need to teach the kid CAD or complex instructions, just sit them down and let them have fun and encourage them to design big ideas they have and share with you. Kids can also design their objects to use for the game of Minecraft. Many designs already created by fellow tinkerers can be downloaded and tinkered with as well.
Its fun, its easy, its educational, its free. Tinkercad