I got paid $10 an hour to babysit when I was 16... 20 years ago! I think $15 sounds more reasonable. Around the same age I was paid $10 per hour in a call centre and $8.50 an hour in a cafe. Yes babysitting is cash in hand, no tax, but at the same time it's far more responsibility than working in a McDonalds. 'Just keeping them alive' is actually a pretty big responsibility, and not one many people (especially teenagers) would take on solo in many other kinds of work.
My partner just got a promotion at work- his workload hasn't actually increased in terms of hours put in or how hard he has to work, but he now has far greater responsibility thus the company pays him more. I don't think babysitting is comparable to working at Macca's purely by looking at the workload, you also have to look at the differences in responsibility.