After repeatedly saying no, you needed to be clear.
She chooses NOT to spend money on childcare and YOU choose NOT to provide free childcare.
A perfectly reasonable position for you to take.
Some women object to paying to have their children looked after and won't hesitate to impose.
Spelling it out clearly to her is the way to go.
Amazing how often these CF types become friends with a useful SAHM that they imagine might be a soft touch.
I had several approaches over the years from parents that needed a " weekly favour" from me, who was just another mother who lived in the general area that they barely knew with a child in their class.
I never entertained it for a second as I was far too busy with my 4 children 🙄.
You'd be amazed at how small they considered these favours to be.
Funny how small they are when you are asking and not doing!
I have definitely happily helped out, and even offered the non CF type parent, but those that are, not a chance.
Stick to your position OP.
She can pay but would rather use you instead.
She's not a friend and I suspect she won't be if you make your position NOT to be used, crystal clear.