An acquaintance from a playgroup I take my daughter to has asked me to watch her son for a few hours tonight. He isn't potty trained, still wearing pullups, which isn't an issue. Him and my 3yo daughter get along for the most part. His mother asked me how much I would charge an hour and that she will gladly pay me, but I haven't babysat in years so I'm not sure how much to charge. I want to help her out and not be too greedy, but I also dont want to sell myself short because toddlers are a lot sometimes too (at least my daughter is). What would be a reasonable wage to tell her?

EDIT: Thank you all so much for the great advice! I think I've decided to go with 15 an hour because that seems like the most popular average rate.