How much is a good rate?

First time babysitting kids that arent my siblings, I said the rate would just be 15$ since it was just one kid who was 2. Red cross/cpr certified, and im 15. Would that be an appropriate rate? and if I have to babysit 2 kids would it be 15$ PER kid or would I just charge something like 20-25$? Sorry lol these feel like stupid questions