I am a grandfather who has been involved in regular child care for grandchildren throughout their lives. (They are currently top end junior school, so drop-offs/pick-ups and (some) weekends/holidays are what it amounts to now.)
From pre-talking all the way through toddlerhood (with, then without, nappy-changing, naps, etc., etc.) to nursery, infant school, and on. Dressing, cooking, feeding, changing, bathing, playing, reading, talking, listening, hugging, drying tears, managing tantrums, sharing lives ... but only part-time and hence with plenty of time and space to re-energise and live a separate life as well.
It has been - continues to be - an absolute joy. A treat. Children are so interesting when you have time to spend with them. Add in love given and received; there is nothing more excellent for an old man's life. So I have found.
I know some people do not wish to look after grandchildren; find it a chore, whatever. Such people - I know many, have discussed at length several times - are just different. I feel sorry for them for what they miss, but, well, people want different things. So be it.
One time not too long ago, I was sat in the nearby town centre, nothing to do, just watching people. Weekday, nothing special. Generally, people looked glum, distracted, other things on their mind; mostly not unhappy-looking, but not happy-looking either; generally we might say just not fully alive in their present moment. Different ages, types of people, all much-of-a-muchness. With one exception: single old people with a toddler in tow. Not the mums or dads: the grandparents. They, ubiquitously, smiling, full of joy, engaged with the world they shared with their charges. If you get a chance, have a look around. You'll see this if you're quiet.
This grandparent/grandchild relationship developing out of childcare is super special. Of course we can see how it is evolutionarily explicable. But that does not make it valueless. Far from it.
Best thing in the world, looking after grandchildren? Maybe not. (Slow afternoon sex in springtime with long-established lover and a modicum of appropriate recreational drugs is my personal number one; then there is having a book you wrote sell well enough to make a little money ... I know others enjoy other things too; chacun à son goût as ever.) Looking after grandchildren is in the top five, though, at least, in my opinion.