Raising my head above the parapet here and will no doubt get flamed, but I was paid for minding my grandchildren. I had them from 3 months of age, 4 days a week from 7 am until 6pm.
I collected from their workplace, fed them, took them out, bought my own nappies etc and my daughter paid me £400 a month. When she had her second child, I had them too, without increasing her payments. I overlapped the 2 for about a year. That was my job, any ad hoc babysitting was done as a grandparent and obviously I wasn’t paid for that.
I gave up 6 years of my life for them, I met all expenses during the week, so soft play, toddler groups etc all included. I had bottles, highchairs all here, bought by me. I have a wonderful relationship with my daughter and my grand children. I am now disabled but they are now in school that provides wrap around care.
We met a need for each other, we were both happy with our arrangement. It also meant that if one of the children were sick, I still had them, so both parents were reliable employees too.
It will only work if you are really honest with each other, if it becomes too much for her, she has to say something. I think you also need to discuss school holiday arrangements too, unless you are working term time only. Maybe start with a trial period so no one feels forced, works for you too if you don’t think it’s working there are no hard feelings.