My mum does this, she has looked after the children twice a week and then once a week as they got older, and when I come in, the dinner is on the table, she's done the washing up and perhaps a load of washing. She doesn't clean the whole house, just potters around a bit whilst waiting for them to get in from school.
I did a non-well paid professional job for years and I think without my mum's support I would have been unable to continue. She would also come over if one of the children were in.
It's not intrusive to me, but we are very close anyway, talk everyday and so nothing in our house is a secret. She is also a sensitive person, so wouldn't hang around when not wanted or intrude.
She's basically a nice person who has an extremely busy life with friends and holidays, but also made time for us. She wouldn't stop doing anything if she needed to go away, just have a holiday from all of us, it wasn't an obligation. My children adore my mum more than anyone, I think (as she also brings them food, treats, sometimes clothes)!
I'm glad it's like this for us anyway and I think it's stopped me getting too stressed/depressed over the years, I know there's always an additional person there to step in if necessary, I don't feel alone.