Babysitting Niece an Nephew....should I have to take them places everyday....?

I got stuck sitting my niece and nephew for the summer. They have a pool out back, lots of toys and video games. My mother-in-law keeps nagging at me to take them places constantly. I took them to a puppet show on Monday at the public library and to get ice cream today. She wanted me to take them to a museum... show more I got stuck sitting my niece and nephew for the summer. They have a pool out back, lots of toys and video games.

My mother-in-law keeps nagging at me to take them places constantly.
I took them to a puppet show on Monday at the public library and to get ice cream today. She wanted me to take them to a museum that we have all visited a lot & to the library AGAIN- and neither child wanted to go.
Gas is almost $5 a gallon and I drive an SUV. I am getting paid $40 a day. I watch them from 7:30am to almost 6pm every day.

Should I have to pay out of my own pocket to take them places and keep them entertained with field trips? I wouldn't mind if it was something fun that their parents paid admission for, like a water park or something.
What do you think?
My MIL nagging me is really bothering me. She is the one that roped me into babysitting them because she didn't want to, and now she is trying to dictate activities!
Update: The boy is 13 and the girl is 7 and I also have a 19 month old daughter that has to have a nap every day at around 11pm.
Update 2: Thank you all so much for your answers! I feel so relieved to hear them! I thought that everyone would say that I was getting paid to entertain them! We do go swimming every day at 3pm so that the sun is not to harsh.
I felt really bad about accepting the $40 a day until I read all of your answers. Thank You!!!!!!!
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