Let me say, @Lilacbluewaters that I do NOT see you as cold-blooded or tight-arsed or mean. I happen to be the eldest sibling (of quite a lot) and back in the 1960s, I watched one of my uncles (mum's brother) and his wife, realised that kids don't eat for free nor create much wealth.
It was watching him and his wife that I took in a well-learned lesson. Whenever K & F took their kids to a family gathering they would bring a cooler-box/Esky well stocked with fresh foods and milk/dairy products ... Uncle K was a milkman in the day. If relatives Including grandparents invited his kids to stay on for a few days, a week or whatever, the contents of the Esky/cooler-box were handed over to the family of the people getting my cousins.
Uncle K commented that he knew kids ate so he made sure he catered for a slab of their upcoming food needs.
Later on, as he aged and prospered he'd leave $5-$10 per couson per day with the "receiving family".
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Come the 1980s, my mother loved seeing her grandkids OFTEN, very often and she would even pre-offer childcare for days my (then) wife and I were at work on weekdays. We would routinely give my mum $20 whenever one child stayed their daily or overnight or $30-$35 if both stayed.
Mum hinted repeatedly that she'd do it for free just to enjoy the company of her grandkids but we always said, like Uncle K, that no-one should be out-of-pocket by much for being 'nice' to our kids and us.
We went without a lot to wind our mortgage down but we kept up this standard for over 15 years.
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One day, one of my less grateful siblings asked aloud at a family gathering why mum would not pick his kids up from school (15km or 10 miles) 'at speed' when he and his wife were at work. He then brought up my name and said, "Well, you seem to be so keen to have Iain's kids at your house and take them places."
Mum, while being diplomatically quiet for a few minutes, got sick of the verbal attack on her generosity of time (and perceived favouritism of me or my kids), finally said, "well, Beelzebub, you ought to know that Iain and his wife drop off money and groceries whenever his kids stay at my place for daycare; you and Mrs Beelzebub do not. I stiil pick your kids up occasionally but not every time because it might be testing my pensioner's budget."
After that, brother Beelzebub stayed under his rock for about six years. I smiled inside at my mother's frankness. And, to this day, I try to pay my own way.
So, @Lilacbluewaters , you are okay to ask the parents to consider some monentary kickback or a grocery card. When my eldest grandson (19yo) drops by and gives up 30-45 of his time to clean my drain or wash my car, I slip him a $20 grocery card so he can buy something 'extra' (above his frugal budget). And, n.b. he'd do it without the grocery card or a couple of bottles of Guinness because, like most of his family, he understands helpfulness and selflessness.
Do not be put off by the understandable but different comments of other posters, @Lilacbluewaters . Their opinions make sense to many.