I see you man, we created this project for the exact same reason. Instructions will be posted, with pretty pictures! :-)

I would start with arduino, not raspberry pi. I don't think raspberry pi's are very good or fun as "maker" projects and end up being too specialized or overwhelming because of the OS.

I've learned more and had much more fun making little toys, programming basic displays, getting sensor data, etc with arduino and other microcontroller development boards. There is too much already going on in a raspberry pi for it to really be a "blank canvas" for maker projects. Hence you see all of these projects where they are essentially repurposing the existing rpi, not creating their own thing with it. Its not really a "maker" project. Its more like building a computer.

Plus, you will find way more true maker projects targeted for kids for microcontroller development than you would for raspberry pi.

Plus, Arduino has a specialized c/c++ library for it which makes things just awesome for starting out. It's like writing python - stupid easy.

Check out the decently kid-friendly kits that Pimoroni sells. They have kits for various levels of difficulties, including some using Raspberry Pi. (https://shop.pimoroni.com)