The author of this article nails it: there's nothing quite as motivating as adding "Poop" to the title of a real web page. Also, you can Inspect Element and get what is effectively an IDE. Is JavaScript the best language? No, it's inconsistent and confusing. But, so is English, and a lot of people learn English first - because it is practical to do so.
Another option that can be useful are Google Sheets. If they are a certain type of kid, they might like logging all of their toys in a spreadsheet and finding the total of the toys and making charts. Of course, most kids will find this boring... but you can also use Google Scripts to do something like scrape a subreddit and store it in Drive [1], which could be fun.
However, why not start with Scratch, or Snap! [2]? They are powerful enough (especially Snap!, I think you can define new data types in it...) and so much less intimidating.
[1] http://ctrlq.org/code/19600-reddit-scraper-script [2] http://snap.berkeley.edu/