Imo the original post is a little exaggerated. Landowners generally don't owe duties to trespassers (other than not to affirmatively harm them), so if you get injured as a trespasser you can't recover. Only issue here is the kid being under 18. Generally, you are obligated to care for trespassing children if there is something on your property that attracts them. Outside that, I don't see why you would owe a trespassing child an elevated duty of care. It's not like you can reasonably foresee a child will trespass, and that's what parents are for anyways.
There may be liability if the landowner knew kids frequent the area and did nothing to prevent their injury. That being said, that whole doctrine is for "kids" and I'm not sure these would qualify as kids.
In any case, it looks like the stairs were a school, and you can't sue the government most of the time, so here the landowner is not liable even if the kid died on the fall.