Am I undercharging?

I (18F) have been babysitting for about 4 years now. I'm CPR certified, I've taken babysitting courses, and I consider myself pretty responsible. When I first started out, I just babysat for families around the neighborhood, the majority of who only had 2 kids, and so I charged 10 an hour. I've never really considered how much people in my city charge, but recently I've been thinking about my rates. I work 3 days a week as a mother's helper, for a family with a 3 year old and 6 month old twins, and when I first started the job, my parents said I should charge a bit more since there's more kids, and so I decided on 12 an hour. Recently, I started working for a new family, and they gave me much more than what I'd usually ask for, and said that I am severely undercharging, and need to up my rate. I guess now I'm wondering what a good rate would be, and how to have the conversation with families I already work with about how I am upping my rate? Any advice would be greatly appreciated, TIA.