I totally disagree with you that that scenario is nonsense. The fact that you assume that if there was a fire it would start downstairs in the furthest away corner of the house, is what is nonsense imo. I will give you a few examples of my points of view.
Many housefires start in a kitchen, and if, say a chip pan, went on fire, the flames would become ferocious very quickly, easily reaching the kitchen ceiling, if the children's bedroom was right above the kitchen, well it doesn't bare thinking about.
What if there was an electrical fault in the hall, at the bottom of the stairs, and the fire immediately started climbing the stairs - that open window might be the only chance of the children escaping...
What if there was an electrical fault in the socket next to their beds, and it went on fire?
Obviously, if any of those scenarios happened in the middle of the night when the parents were asleep in the next bedroom, and if for some reason, there wasn't a smoke alarm in any of the rooms, then tragically the poor children could still be overcome by the smoke, or even the fire itself. But if their parents had just been sleeping a few feet away, there would at least be a chance that they either heard some noise caused by the fire, or their children screaming... If the parents were outside, 2.5 minutes away, they might have never even got the chance to save their children.