All you have to do to find this out is look at the first chapter of the book. The boys are on an airplane heading away from England to try to get them out of danger -- there's a war going on.
But the plane is shot down and crashes on this island. (They were headed for Australia, which is how you end up crashed on a tropical island if you are a bunch of people from England.) The only adults on the plane were the pilots, apparently. And they died. So now it's just the boys.
Yes!, thats exactly what I thought the answer was. Just wanted to make sure. Thanks! =)