I care for my twin grandsons every other Saturday for the day, and we make it a fun day doing baking, playing games, visiting the pet shop to admire the rabbits and guinea pigs, and then the park, and generally doing the sorts of things I enjoyed with my grandparents. I also pick them up from school every Monday, give them tea and get them ready for bed. They have a Friday night sleepover with me every few weeks.
My daughter's MIL cares for her twin 15 month old girls 3 days a week, and I call round to help with them once or twice, then mind them for a couple of hours when my daughter is at home with them, and do a bit of baby-sitting. MIL went part-time in order to be able to care for them, so is compensated for her drop in earnings as a dinner lady and after-school club assistant. It works out much less than paying a registered childminder and everyone is happy.
My newborn grandson is further away from home, but I'll see him as often. My son runs his business from home and DIL is a stay-at-home mum, so childcare won't be needed, just babysitting.
When my eldest grandson was younger, he stayed overnight every weekend, and I used to pick him up from school quite often. I see him every week when he is staying over with his dad.
I haven't looked after any of the children as a way to help mums and dads go to work, but if I had been needed, I wouldn't have expected payment, just the occasional 'thank you' and treat.