Do you think this a fair babysitting rate? INFO?

I am a thirteen year old and live in AZ. Although I have never formally babysat for another family, I do watch my niece who will be one this July, and my sisters friends, about eight or nine. I am CPR certified. In general for families is this fair, $8 an hour for up to three kids + $2 an hour for every additional kid? If not, what do you think? I did babysit once last Wednesday for a five year old, a three year old and a ten month old from 4:45 PM to 10:30 PM. I played WII with the older ones, cooked mac n cheese, cleaned better than when I got there, bathed all three children, made the baby a bottle, watched Little Mermaid, read them a chapter from Ramona and Beezus and put them to bed. The mom was aware of this and gave me $12. I did not say what my pay was because she did not ask. Next time what do I say? Any tips for future jobs? Thanx a milly!