I also find an accident and subsequent cover up hard to believe, maybe because in my mind I can’t imagine my child passing away from a freak accident and then thinking “I’d better cover this up” instead of “omg call 000” But then I’m not in someone else’s position.
what I do feel is more conceivable (not that I would do this) is if the accident was caused by someone - eg pushing someone and then they accidentally fell down the stairs, broke their neck - that sort of thing. And you suddenly felt very responsible for the death, leading to the need / want to cover it up.
Again though, it’s so hard to put yourself in these situations and think how you’d act/behave.
I just hope LE find something, find William and give the little man the burial he deserves.