Teaching a teen to create a blog/website and then flipping it to clients is a great way to earn much more than they could in most venues.
If you want to find good products to affiliate, check Clickbank's highest "gravity" items - these are the hot products that are really selling/making money.
Teaching your teen to rank for a low-compete keyword phrase, and matching it to an affiliate product (Amazon's probably the best and easiest for your teen), and then making sales that way by driving traffic to the site via backlinks and social media, is a life skill they'll be able to mine now and in future.
Adsense would be another form of that (try the Micro Niche Adsense Master's course for $77 - you won't regret it, and it works)...
If they learn to generate backlinks, this is also a service they can offer on Elance, Guru, oDesk and the like, though they may not be eligible unless you run as the front man.
Just some thoughts. (Personally, I freelance write on Elance and make a full time income.)