I once talked to an animal control officer. He said if he finds a dog in a closed-up car on a hot day, he will break the window to get the dog out, no matter what.
He said within less than 20 minutes the dog would be dead, first it would slip into unconsciousness in a short time, and be dead not long after, just from the heat, even if there is air.
So I can believe these kids (if they put themselves in the trunk that is) were unconscious in a very short time, especially since there were 3 of them in such a small space.
However, reading that one of them had a plastic bag over his head takes this to a different place.
Just to stay real.... a closed car in the sun gets hot because of the windows.... a trunk wouldn't get as hot as fast.
I am sure it was uncomfortable, and would certainly be as hot as it is outside, and I don't know how well they could have gotten air in there, but the situation of a trunk is not like the situation in a closed glass greenhouse of a car.