Tbh tho, I had a crappy childminder who thought nothing of springing an early finish on me, or just cancelled a day due to taking her own kids somewhere.
It was only 1 day a week I used her for and even so it was massively inconvenient when she just dumped this stuff on me. That said she turned out to be really quite awful as it goes.
if I were ever lucky enough to be a grandmother, and I heard my dil refer to me as "being very good to us" and "i appreciate her coming over" in this us vs staff way I would be in bits.
I have friends who both fly, they pay (her) parents a fixed sum a month and it helps them rely on one set of their parents to do the wrap around care they need. It helps everyone feel better about the arrangement.
Your mil isn't taking the money because she doesn't want the commitment to have to cancel any plans she wants to make. Your demeanour may also be at the heart of her involvement balance.
The only yanbu in this is if she gives you no notice at all of her plans, because clearly there is an understanding and you do need notice to arrange back up sometimes.
You are in between a rock and a hard place here, she wants to help, but on her terms. That's kind of fair enough
I'd suggest you have an honest conversation with her and say that you done want to take advantage of her, and it's great if she does want to come down and help out here and there, that she's not obliged to, but she's welcome to come anytime she likes.
If I were you I'd get a childminder to do the care you need, and if mil wants to come down she can.
Yes you'll have paid childminder fees, but it changes the dynamic entirely.