This is a big deal if she isn't reliable. My mil covers for 4 hours a week. HER choice and preference. But she us late wuite often. Often fails to give notice that she has decided to take a trip and can't come. But when I said I would be looking at other options had a fit about not being able to watch our kid on "her" day. She didn't get that I couldn't blow of work at the last minute because she felt like going to the beach.

I'd just ask and add that she can have some time to think about what she wants to offer and let you and your husband know.

When she doesn't do things the way you'd like/prefer, don't jump down her throat.