What actually happens is that the HOST PARENTS supervise the group. So if your kids are invited to a friend's birthday party then yeah, you don't stay. I assume that it's the same in the US and pretty much every other country.
We also don't leave our four-year-old kids in the car alone. Well, some people do, which is why the local newspapers have been full of stories about the parents getting arrested for it and social services getting involved.