Short answer: Seems about right. You could probably charge more, depending on the family.
Long answer: I’ve also been babysitting for 10+ years, and my full-time job involves working with children and youth. I’m paid on average $15 an hour, but I charge/accept less if it’s only for one kid and he/she is asleep for most of the time. On the other hand, if there are 2 or 3 kids and they are awake and mischevious as children are wont to be, I often make $16-18/hour. On short notice or for very long periods of time (5+ hours), the families I sit for have paid $20/hour.