This has to be a personal choice between parents and grandparents. My in-laws would be insulted if I'd offered to pay them to watch their beloved grandkids! But my mother, who actually could use the money, would've welcomed it. I'd say if you are talking about babysitting while you go out, a one-shot deal, it'd be nice to offer or bring something over for them, maybe offer to buy pizza for dinner that night. If you are talking about ongoing care, say every day after school, then open up a dialog about it. "Mom, Dad, I know it's a lot for you take care of the kids..." and broach the subject.