My siblings sometimes leave their kids at my parents' house for days just for the fun of it and so they can get a break, and I sometimes go there too because nothing like reaping the fun of being with kids with none of the responsibility of being a parent. What's not fun though is watching my parents feeding them sugary bullshit all day long.
It's insane. Ice cream for breakfast, cookies for snacks, cupcakes for dessert after a dinner consisting of chicken nuggets, more ice cream, and chocolate pudding or sweetened yoghurt before bed, all in one day. It's got to the point where the kids will scream and cry for treats if they go without any for more than an hour.
"Mom the more sugar you give the kids the more they'll demand."
"Well they're at grandparents' so they get some extra treats teehee!"
"I don't think they should eat ice cream twice a day."
"Oh cut it off, what's important is that they eat!"
It's fucking miserable to watch. I have never seen my parents offer them some fresh cut fruit (of course if they offer it at this point the kids will cry and throw it on the wall because duh it doesn't taste sweet anymore because you ruined their palate at the ripe old age of three years old). The whole "teehee don't tell mommy about this!" drives me nuts too - way to teach a kid to lie and binge in secret - as well as the "well they're kids, don't be a killjoy, they can have some" no they can't!! They're the size of a backpack and you're feeding them amounts of sugar that would be harmful to a grown ass adult. Kids should have fruit or home baked, refined-sugar-free stuff like banana muffins for treats, end of. Aaaargh.
TL;DR: Don't leave your kids alone with people who will shamelessly wash all your hard work raising healthy kids right down the drain.