I haven't read all 4 pages of the replies yet.
My kids are teenagers, so I'm not in the grandparent category right now. BUT...
When ODD was a year old, MIL moved from another state to here, and lived about a 15 min drive from us. She decided to quit working/retire. She offered to babysit full time, free of charge. That was awesome at first because it saved me about $800 a month on full time daycare at the time (ODD is 16 now).
Within that first month, she started to complain about how we weren't paying her. And I had to supply her with all of the baby bear...toys, a pack & play for the kid to nap in, food, diapers, wipes, everything. Bought a car seat for her to use for ODD, too. Also left an extra stroller at MIL's house so she could take ODD out on walks to a small park nearby.
MIL never took the kiddo outside. She stayed indoors all day. not even in the backyard. Never had social interaction with anybody else.
On top of that, MIL complained that she wanted to go away on vacation. This meant that DH or I would also need to then take time off of work...PTO which we literally did not have. Then MIL said that whenever ODD got sick or even a cold, she wouldn't babysit. So we had to take time off of work for that, too...stuff we'd had to do anyway when the kid was in regular daycare.
Then she demanded to be paid what we were paying the daycare before. I went along with it for another month and then told DH that I was calling the daycare to try to get ODD's old spot back. It took 5 more months for a spot to open back up.
It was hell.
Never again.
For years afterwards, MIL would whine and offer to babysit for us so we could go to the movies or something every Saturday. We did that 2 times, and then within a couple of days of that occurring, my MIL demanded that my DH take HER out to dinner and a movie. So I said, "No thanks."
DH & I didn't have any adults-only dates from then on unless it was during the couple of times a year that my sister would fly in from the UK.
If I had to do it all over again, I would have never agreed to all of that. It was a disaster.
If you are going to agree to babysit your grandchildren and if you expect or want to be paid, then negotiate that up front BEFORE THE BABYSITTING BEGINS! Know that the grandkid(s)' parents will be relying on you to not flake out, to not just change your mind on a whim because you've decided that you all of a sudden want to spend 3 weeks on a holiday.
If you want a foot loose and fancy free lifestyle, then DON'T BABYSIT YOUR GRANDCHILDREN where the parents have to rely on you for regular childcare so they can work.
If the discussion of the parents paying you NEVER came up before you started the babysitting, then you SHOULD expect some frustration and resentment on the parents' part.
If at any time, you decide that you need to change the timing or frequency of the babysitting, know that finding reliable childcare is hard for parents and it might take them awhile to find a suitable replacement.
If you're not ok with any of this, then ONE SHOULD NOT AGREE TO BABYSIT!