mattwnz:

 

djtOtago:

 

As a parent, I would be more worried about my kid cracking their head on the sharp edge of the stair tread if they slipped on the stairs more than I would them falling through the gap. 
or running into the steel beams holding the stairs.

 

 

 

 

An edge of a stair is unlikely to kill or cause serious injury through compared to a fall from a height, through the stairs. 

 

which one is more likely though?

the latter one would only really be applicable for a young child, less than 5, the former for anyone who can walk.