Teach kids Ruby and if they are interested they will learn other languages later. Ruby (before you go on to rails) is very flexible in its use and good to branch from. C++, javascript, and opengl have very particular use cases, which is fine if you know thats you want to do, but pointless if you just want to learn generalised programming concepts...
If you want to go lower-level better to learn some assembly (or at least C) instead...