I don't get this story, really.
Why did the grandparents call it an ' accident' . We've had an accident, they say. Surely, f he's ok, and they think.ponds are safe, which they clearly do, or they'd have fenced it, why didn't they say, ' Little Johnny, stepped in the pond - going after his car - but we lifted him out and gave him a good bath and he's fine' . You don't alarm parents by using the 'a' word like that, if all's ok.
As for messaging later and saying it was a 'genuine accident'. Good grief! What the heck else could it be! You'd say that if there was a possibility of him having been pushed in by a sibling!
I've had too much lockdown, and am seeing fake stories everywhere on Mumsnet, this week, but how do you fall in feet first? Going in feet first would be stepping in to get a toy back. Small children's heads and torsos are heavier than their legs, that is why they tend to topple forward on to their front or face - extending their arms. Also, 'dropping' (?,) or maybe jettisoning a toy into water would take longer than a few seconds; the kid would be on the edge looking and feeling, or having a few attempts to throw from more of a distance.
I hope it isn't offensive to wonder if a story isn't true. But anyway, hopefully, the thread has raised awareness of a really risky problem in gardens. All the advice about grills and wrought iron covers is fantastically helpful.