Paying grandparents

My stepmom recently brought it up to me that if I ever need them to watch my daughter for a couple hours that I should pay them. She said that it’s a lot of work for them to watch my 2 year old. I don’t ask them to often and when I do it’s because it’s been weeks since they’ve seen her. I haven’t brought this up to my Dad because I don’t know why to say. How much would be reasonable? Should I do $15/hr? They sometimes will have her for a whole day when my Dad says he wants to see her.