Old people are expected to die, young children’s deaths are far more noteworthy as can be evidenced by the hundreds of reports of childhood death by accident available to read online.
And some of those are under 5's suffering fatal falls. Public Health England have the figures. The link to their data is in G-Unit's link. It's rare but happens. There are about 3 cases a year in the UK (fatal falls in the home in the under 5 age group).
Falls are the most common accident in the home.... and a fall even from 4-5 feet can cause unconsciousness if your head hits a hard surface... and
the first thing to assess in an unconscious person is their airway. It's thankfully rare but you can't claim it doesn't happen. I'm actually stunned that people aren't aware of the risks of leaving toddlers unsupervised at home... but I'll admit to getting a bias view working in a hospital and seeing unconscious children lose their airway.
I'll tell you what I consider is nigh on impossible and that's to carry a 3 year old out of the 5A bedroom window!