If parents are PRs or citizens they can care for children but visiting grandparents are caring for children versus yo7 paying for childcare which is why it is considered illegal work. Grandparents have been refused visas if they say they are coming to cook, clean and provide childcare. If they say they are coming to provide emotional support and enjoy the new baby that isn’t an issue. Would order GCMS notes if you want to see why you were refused. You made it very clear that your parents were taking the place of after school paid childcare which essentially indicated that they would be working illegally.