How can I kindly tell a friend that they need to pay me to watch their kids? I’ve turned down payment in the past because she’s a single mom. Now I feel taken advantage of because she doesn’t offer to pay anymore and is the only reason she calls.
I would just say I'm busy (don't be specific)...or if she asks about such and such date, have her wait while you check your planner, and of course, you have things on for that day too. Those things could be reading a book, watching a movie, whatever. It's your time. Just don't tell her. However, if it's really important to her, you could cancel what you have planned for whatever $$ you think it's worth to watch her kids.
If she only wants to talk to you to get you to do stuff, she isn't much of a friend. If she tries to make you feel guilty, she's a parasite. That is what vile people do - take advantage of kindness, offer little or nothing in return and then play upon guilt when they don't get what they want.