Family babysitting

Hello, do any of you have family babysit? My mom or dad will be watching my little dude for 5 hrs 3-4 times a month each so like each parent ( divorced) will watch him 5 hours a week while mine and my husband's schedules overlap. They don't expect to/ want to be paid in any way for watching him. I feel bad having them do it for free! Should I try and offer money again? Anyone have grandparents that help out just because they love you and their grandchildren haha