Am I being awful stipulating a minimum about of hours i will charge for irrespective of hours attended?

I have a pain in the rear mum who thinks I am a babysitter and no amount of asking, telling, explaining, pleading or chasing will make her understand.
I have bent over backwards to accomodate this mum and she is still taking the micheal!

does anyone else stipulate minimum hours? What about retainer?

Again I have bent over backwards to help her out (too my cost) with the retainer.

I have sent her a letter informing her that I will be reviewing the contract, invited her to an appointment on the 17th August (or another date around then if that is not convienent), I have given her written notice of change of fees, change of retainer fee and implementation of minimum hours. To be discussed and agreeded in writing at appointment, when I will give her written confirmation and it will be implemented on the 10th Sept.

Am I being fair enough?

Bit of background (perhaps I should have put this in first lol), it has taken me 5 months to pin mum down about sons adhd, she texts me on a sunday round 9.30pm for hours for the following week, this month I minded the kids for a total of 5 hours, 45 mins one week, 2.5hrs the following and then the remainder the following week. I knew the hours were variable but not this minimal.

Thanks (sorry if I have rambled on)