My OH and I are 66 and at the moment we mind our 2 year old grandson 2 days a week from 7.30am - 5pm; one of his parents delivers him to us. His mother works 3 days a week and the other day is covered by the "other grandpa" doing 2 days a month and my son using annual leave for the remaining days.
On one of our days we stay at home and do the usual house-based activities; he "helps" us with the houswork, garden, feeding hens and other pets, baking, playing music and so on. We always have at least two walks as well.
On the other day we go to a museum/park/beach, depending on weather - we take picnics.
We take him home at 5pm. We're lucky that we're fit and active - as are most of the grandparents we meet when we're out and about - but there must be some who are really exhausted, or who don't have transport or money to go anywhere. I don't know how they cope.
I'm fully retired but my husband's an academic and still does a few days' work a month - sometimes from home, though he travels too. I have four children and I anticipate that the others will have families in due course. I simply don't know how we're going to be able to help them all, and of course it's an expense - I don't think we could stretch to doing the same for them all, and that is upsetting.
I know lots of people of our age who're in the same position; their own children aren't having their families until they're in their 30s. By that time, they have huge mortgages and outgoings which mean they both have to work so the grandparents find themselves in their late 60s, retired, but unable to make plans for holidays and so on without asking for "leave" - it's a complicated business.
I love the fact that we see so much of our grandson - but there's no doubt that it changes the relationship. Sometimes I long just to be a granny and not a substitute parent.
My daughter-in-law would prefer to be at home with her son full-time. I can't imagine what it must be like to feel like that and have to hand over to granny a couple of days a week; I was at home until my youngest went to school. We do our utmost to do things their way and make it easy for them - luckily we're on the same wave-length most of the time, and I think it's imperative that we do things their way for the child's sake. If you're an occasional granny, maybe you can be a bit more relaxed and indulgent, but if you're actually providing regular child care, it's more like a job.
(Unpaid, and without lunch breaks!)
My apologies for the long inconclusive post. I don't have an answer.