How do I kindly tell my friend I’m not babysitting her child for free and she needs to stop acting like a charity case?

Additional info: I 23F literally can’t stand you g children (think like under 7). They scream, the ones in diapers need changed and the un-potty trained ones pee everywhere.

My friend 22F and her long term partner 28M have a child 1 1/2M together. My friend recently lost her job and her bf works overnights. My froend is super inconsiderate and schedules things during the one hour a week I have therapy and always guilts me into watching her kid while they donate plasma.

Then because she lost her job she asked me not to charge my low cost babysitting fee ($10 an hour. Most sitters around here charge $15-20 per hour per child).

Now she’s trying to guilt me into just doing therapy while here child is here but I am not talking about my trauma in front of a 1 1/2 year old who is learning how to talk.