OP has said the house is basically getting trashed in multiple rooms with her daughter getting lots of things out and not putting them away in 2 and a half hours. It's not just a cup or two. I was thinking the cleaning might be too much, but I'm getting a feeling that there's something else going on here.
Like I had a bag of my daughters old clothes on a bag in the kitchen ready to take to charity shop and while my mum was here my daughter must have decided to go through it and put them on her teddy bears the floor was covered in clothes. Plus toys, dirty dishes etc.
She leaves Legos and magnatiles and colouring stuff strewn around and counters covered with stuff. OP has the house very tidy in the mornings. I think she needs to ask her mom if her daughter's being difficult about putting stuff away and her mother's having discipline issues with her. It's apparent the mom, for whatever reason, isn't directing the daughter to clean up one activity before they start another. I'd want to know if my kid wasn't listening to the person I had watching her.
She also said her mom will not take this conversation well. Her mom was the one that approached her to do this childcare for money and retired to do it.
My mum does not take any perceived criticism well. No matter how lightly I tread it will likely be a big row followed by the the silent treatment for a couple weeks.
She needs to be able to address issues with her mom's childcare with her openly without her mom having a fit. She's afraid to ask why the change and why is her mom letting her kid trash the house daily. That's an issue.
I'd have the conversation though. I might ask her directly if she's tired or having some health issues or is it a discipline issue. But she's paying a nice chunk of money that she could be paying wrap around care and her house wouldn't be trashed daily. It's come to the point where it might be cheaper and easier on OP to use outside care because there's going to be resentment developing. And she knows her mom will kick off if she does that. But in the end she has to do what works best for her family, not her mom and that might mean ending this arrangement.