When we looked after our grandchildren during the week our children used to tell me what our grandchildren should eat, they told us when they should have a nap, they told us not to get paint and glitter on their clothes etc. etc.
We told our children that what happens at grandmas, stays at grandmas and if they didn't like it they could pay for child care.
We were not stupid, we knew which foods they liked, we changed them into play clothes to keep their own clothes clean but apart from that we had fun. Our children knew our grandchildren had fun with us because they were always asking to visit us.
If children want their parents to look after their grandchildren for free then they keep their noses out because we grandparents are not stupid, we know what our grandchildren like and dislike.
A friend of mine looked after her grandchildren and their parents gave them a list of do's and don'ts, she said it was so stressful keeping to the list. Ridiculous. Unless they have allergies then just let them be children. Or like I said, pay for childcare and I am darned sure they won't do that.