I actually settled on Lua for teaching my son.

Very simple minimalistic syntax, few concepts, few primitives, few keywords, dynamically-typed / garbage collected, variable arity, no real gotchas, good tool support, great speed, the ability to access a key in a hash either using bracket or dotted notation, and one of the best programming books for any language (PiL).

But it also scales well; closures, first-class / true anonymous functions, metatables / metamethods, nice simple API for talking between script and C, tail-call optimization, coroutines, short circuiting operators.

And, the path from Lua to JavaScript is very straightforward. In fact I'd say that mastery of Lua would make you a mid-level JS programmer right out of the box...