Most people don't like change much, even when that change is pleasant. Something about routine disruption makes people uneasy. The same is true for children, except, unlike adults, they usually can't pinpoint the cause of their distress. This makes the first few weeks of anything new a cranky, whiny mess...including summer vacation.
So how can families move seamlessly from one routine to another?
First, if you can, structure your summer days so that there actually is another routine to go to. Many times we want to leave summer days open to give them a proper vacation feel, but what we consider a break might be incredibly stressful for our children as they struggle to figure out what to do with all the hours that had until recently been laid out in excruciating detail for them. You have to ease then into freedom. Start by lining up activities and chores for the entire day and gradually introduce ten minute breaks of free time. Then fifteen minutes. Then half hours. Soon enough your cistern should be able to create their own structure with just your suggestions, and then you'll be home free until school starts up again and you have to deal with that change.
Second, do not forget to feed them. Not that you'd starve your kids but remember that school meals aren't necessarily at normal times. My kids eat loch at school at 10:30 a.m. Their bodies are trained to be hungry at that odd time but without the set routine that tells them they're hungry, they might not recognize the feeling and get cranky instead of asking for food. If you make sure a snack is out when your kids are used to eating, you'll save yourself many unnecessary breakdowns.
Third, keep your bedtime routine the same. They'll need a constant in their lives as everything else changes and at least they'll know that your part of their day remains the same whether they're in school or not. It will show them you're consistent.
Fourth, even though they have nowhere to be, when they wake up, make sure they get up and ready for the day. This gives them menial specific tasks to concentrate on as their brains wake up and they won't be drowned in a fog of general anxiety at the thought of an empty day they have to fill with their own ideas.
Throughout it all, remember, school will be starting again before you know it.