Three.js, so very accessible and you can have games load up within 2000ms. You can use the corresponding github codebases - I loaded up his minecraft demo successfully a while back.
Three.js seconded for young children. I followed "Game Development with Three.js" with my daughter when she was around 10, and we both had good time. The approach in the book is very interactive: make a change and see immediate results, this works very nicely with impatient young ones.
