It depends on the kids and their age. With older kids I think it's nice for them to have a chance to just BE for a change rather than rushing from class to class and activity to activity. There's no harm in being lazy once in a while and most kids can make their own fun if left to themselves. I know as a kid I relished the chance to just sit and read for a while on school holidays. On the other hand, if your child is in the habit of doing dangerous, reckless things like lighting fires when left to themselves then they're going to need some proper supervision.
My kids are little (not yet school age) and I don't have a paid job so school holidays are much the same as usual for me except that some of the stuff I'd normally take them to is closed and there are other different activities on during the holidays.