If you would like her to work less this week and more the next, you pay her for her normal hours this week. And, pay her extra for her extra hours next week.
You can't be changing her hours and pay without consultation. It's just rude.
You can't assume she can afford not to work those hours, you can't assume she can pick up extra to cover those hours. This is your choice not hers, so she should be compensated appropriately.
I cleaned for a year when I left a difficult relationship as it fitted around small children and suited me at that time.
I had one lady who thought nothing of telling me she only needed x hours that day if she'd been able to do a bit herself. She didn't factor my drive to her into her thoughts, my time and petrol travelling and my pay being down x amount of hours. I couldn't easily take on another slot without major replanning of my working week.
I was so cross. I left after the second time she pulled that stunt and found another nice family to fill the slot. She was quite upset and kept sending me texts asking for me to return. But I could afford a flaky Cleaning position.
When you are on benefits you have to earn a set amount and deposit it into your account, otherwise you don't get your benefits and your children suffer. It's very black and white.
Don't mess with the cleaner!
I now have my own cleaner and always pay well and fairly.
Ps, OP, make sure you put aside a decent Christmas bonus for her to make up for this.