By the way, how old is your child? Looking back at your previous questions I think she is almost 10.
Here's the thing I got most. Your mom is not a babysitter. She let your child stay there while you were at work, free of charge. If you'd have been paying her and given her a list of activities you wanted your child to go to she would have taken her.
Grandma understands that kids don't need to be busy with "things" all the time and that kids do need complete down time too.
Stop being mad at your mom. If you didn't like what she was doing you could have hired someone then.
I got paid by the hour by the kid. So if your kid has company you have to pay for that kid too. If you have them take kiddo to and from stuff all the time then you have to pay a gasoline allotment.
I did NOT get minimum wage by the hour because if I got that then mom wouldn't have made any money. She would have only been going to work to pay me.
I was also responsible for paying my own taxes, I was contract labor, a babysitter, not a true nanny that comes in and manages the house every day and minds the kids along with that. That means I was not eligible for any benefits or vacation or social security taxes being held out. Just like paying a plumber or mechanic. You pay a fee and that person is their own business and pays all those things on their own.
You still have the right to file and claim child care by getting that person's information that your tax person says you need. They need to understand this up front. That you are not going to withhold any taxes and they won't have any benefits and that you will be filing your child care on your own taxes. So the government will know they have been working.
Tell them, as contract labor, that they won't have set hours. That you will give them a schedule for the month before the 4th week of the current month has started. If you truly don't know what days you work until the Friday before the next week starts then find out if that's okay.
I would also have several teens on stand by so that if one can't be there another one might have time.
Overall I think this plan sort of stinks.
Why not plan activities one day per week for your mom and your daughter then let them enjoy summer. I would never have my mom watch the kiddo then invite others over too. That's just not right. Unless you're paying them to watch that other kiddo too, same, if not more, than you pay for your own child.
Then your mom will have specific plans set up for her to take your girl to and it won't be something random or off the cuff that she has to plan and implement.
I'd also pay her. She deserves to have a little extra cash. But if you truly can't then let up, she's your mom and not your babysitter. She is being there for you.