It's not as though the bouncy castle was in their back garden and he broke in, though. They put up a bouncy castle (= kiddy magnet) on public playing fields, where kids... um... play, and didn't supervise it. And it's not as if they didn't know that it ought to be supervised, because the hire company had told them. That does sound negligent to me (plus wasn't it found on the balance of probabilites that the Perrys had actually invited Sam to use the castle? I'm a bit hazy on that aspect, though).
If you try to eliminate risk it wll mean cutting out a lot of the pleasure of life. If you try to sensibly manage risk it will just mean applying some common sense (e.g if you are specifically told that something needs to be supervised, then supervise it).