Lua. Computercraft is a very mature platform for playing around with, and is powered by Lua scripts. Lua is a very simple language to learn, and Minecraft is a great platform that kids seem to love, and the tangibility of seeing the world they create be molded by the code that they wrote really has a profound effect on kids.

At least in my experience anyways. Semantics first, concepts later, this is why Lua is easier to learn than JS.