I know of grandparents who charge for childcare, which is perfectly reasonable in my opinion especially if they are giving up work or the opportunity to work in order to support their own children's careers.
...that said I also know several families where there have been rows over different issues concerning childcare and where children have ended up in paid childcare after grandparents' care has not worked out. I guess that is more likely to happen where grandparents are paid as the families can obviously afford to pay for care and make other arrangements.
My DM has never charged my DB for her 2 long days of childcare...but is retired with a good pension...talking with my DD who hopes to have a baby within the next few years (YIPPEEEEE!!!) and has always said that she wants me to childmind...but I would have to give up work (or could be a nanny with Grandchild....NWOGC!!??) I would need to be paid something....so it completely depends on the financial situation of the Grandparents...
I am aware some grandparents drop their hours at work to care for grandchildren. I think of it as quite a risky arrangement as if parents then choose to use a different form of childcare it could lead to family tension. Also impact on pensions! I wouldn't be averse to paying grandparents if they were in a position to offer regular care, I would have trouble getting my own parents to take money from me (they won't take expenses for babysitting). In summary - parents should offer. Grandparents are at liberty to refuse or accept as they see fit.
It is very different if it is one day a week or full time. For full time care I understand payment would be appropriate, but a day a week to save parents money on nursery (and gp can still work) is different.
I have never paid for care from grandparents, however it was very unreliable, and I now prefer to use paid childcare.
My DGM never took a penny for dd1 even though it was offered - her and dgf loved having dd1 there. However dd2 (18 years later) MIL expects spare ribs and chicken curry and rice with a bottle of wine just to baby sit while dd2 was fast asleep. we don't ask her any more.
Two of my friends childcare their g/children and charge - we don't and its never been brought up, tbh we are happy to be helping them out financially as childcare costs are not cheap. However, were we to be offered some petrol money or a day out occasionally we wouldn't say no!