I grew up with blue flame non-vented propane heaters in every room of our home and 90% of the entire county were the same... I am still alive and I do not personally know of anyone deaths related to CO. One or two windows in the entire house would be cracked open about 1/2". As long as the flame was pure blue, it was considered safe. Any yellow tips to the flames was a danger warning. I have one today in our cabin. The doors and windows are not that tightly sealed, so we do not intentionally leave anything open. WE do have a CO detector and it is tested every weekend.

I am much more afraid of catalytic heaters. I would not run one of those in the house when we are asleep.
I do use a catalytic heater in the deer blind. No open windows, but I do have a 4"X 4" screen vent at the bottom and another near the ceiling.

CO is about SAME as air - location NOT as important - but middle is best (not near floor or ceiling)
propane is HEAVIER than air
nat-gas is LIGHTER than air

CO
density- 1.14 kg/m³

AIR
density- 1.1455 kg/m³

Propane
density- 2.01 kg/m³

Nat Gas (CH4)
Density: 0.656 kg/m³

I have to wonder if the youngster got cold during the night as his father and grandfather slept and closed an opening that had been left for ventilation purposes. So many dangers that kids must be fully aware to stay safe.