JScript is free for windows, with extensive (adult level) documentation, all a free download for Windows at
this Microsoft page. In addition, you get free code samples and technical articles. You can't beat the price.
The kicker is that the language is extremely> useful in building web pages, and any kid who is learning programming will eventually want to learn to make web pages. To use jscript as a straight programming language the kid will need to learn to use a text editor like Notepad and the scripting runtime called Windows Scripting Host.
On the other hand, jscript is more complex than Lego and does have a few gotchas in the language. Booleans, if I remember correctly, have three possible values.