While I agree that the Play Store is becoming more controlled, the fact that you can sideload apps means that ultimately, the platform itself is still very open -- maybe just not as much to the typical end user.
I'm going to say that's not true, because that opens you up to being exploited through methods you trust. You might not think a PDF can hurt you, but without AV you don't even stand a chance.
I think since I'm in high school, I can shed some light on the kind of demand there is for what you're describing. Generally, parents would be happy to sign their kids up for an intro programming class. When I was in elementary school I certainly did at least a few, ranging from a robotics focused one, to one specifically with UDK for games.
I'm not sure how the classes are marketed for parents, but to kids, the thought of building your own robot, or creating your own game is very exciting. Obviously, the reality of the class is that instructors end up spending the majority of their time working out technical issues and helping the kids whose parents signed them up for something they have no interest in. It happens to be much like a typical classroom. I didn't end up learning nearly as much as I wanted from any of the classes, and spent most of the time working ahead with little to no supervision.
I think the main thing you have to decide if you really want to do this, is whether you want to teach the kids who have the interest, and the basic level of experience, or the ones whose parents are looking for a good way to spend a few weeks. It sounds like you, like me, fall into the former group. If you're really passionate about what you're teaching, it will be a much better experience for everybody involved if you can get those kids. I can't offer much more advice than that, only being in high school, but I'd be happy to answer any other questions (or try to answer for my parents).
As somewhat of a disclaimer, I'm not sure where you live, but where I am (Northern Virginia) _many_ of these kinds of classes/camps offered after school, or during the summer. Of course, this has to do with the region, so take that with a grain of salt.
Either way, good luck!