There are some languages designed expressly for kids and learning. http://scratch.mit.edu and http://alice.org come to mind. That said, Javascript wouldn't be a bad choice. It doesn't have a compile step to go through before you can see your results, and is great for sharing (put the code on any web server and your kid's friends can see their creations).

I've toyed with the idea of a Javascript-based language for learning -- some thoughts on that here: http://contraterrene.com/blog/?p=136