From laying on them the same as cats do. The autoposy said they were asphyxiated by the snake but it did not say how. Common sense takes over when dealing with an animal that size. I've seen 25' long Burmese Python take a goat. When he constricted and tightened it sounded like someone stepped in glass from the bones breaking inside. After that it was only a matter of seconds before it stopped moving then about a minute later the snake relaxed then proceeded with her supper. The strength a snake that large has can easily pop eyeballs from sockets and (WARNING GRAPHIC TEXT) your insides would come out the top and bottom holes. There would also be blood around the mouth, ears, nose, anus and eys sockets and the eye balls would be red as cherries from the increase in blood pressure when it cunches down. The brain would show severe hemmoraging not starvation of oxygen. Not to mention broken/crushed bones and dislocated joints.
If the snake did constrict them it would've woken them from crawling into positon. I've messed around with snakes and ither reptiles most of my life as part of my outdoor hobbies. I have had several snakes as pets in the past and no matter how tame they were when not threatened, if they were proviked or felt threatned they always bite while they constrict. If the subjects are too large to constrict they bite and try to escape. I've been bitten by a small python (3') and it is something I don't want to be bitten by again.
I honestly think this snake escaped it's cage and went looking for a warm place after it was out and where they were sleeping was the first stop. It crawled across the boys and most likely it layed on their faces and neck and the rest if their bodies and they suffocated. If those boys weren't there that night it would've crawled into the next warm spot it could find. The excuse they were at a farm would be false as to why it happened. Snakes have amazing smell, nothing on this planet smells like a human more than a human. The smell of animals would not have been on the boys for extend periods of time due to the constant contact with humans and human 'contaminated' surfaces (cars, clothes, etc)