Some searching suggests the drowning death rate for children globally under 5 is 7 per 100,000 per year, lower in the US. Yes, it’s the second leading cause of death of kids, but it’s not a high risk. A child has just a 0.035% chance of dying by drowning before their 5th birthday.

Careful here, this is where statistics can very much lead you astray and you can end up like the statistician who drowned in a lake with an average depth of one meter.

I find it pretty likely the vast majority of kids don't have access to open bodies of water that aren't fenced in, especially in the US where laws tend to require fencing them in. You'd need to break this down to number of kids who actually have opportunity to drown in an open body of water. You can't count kids living in a desert with no pool towards the risk factor of drowning.

Drowning is a real risk for any non swimmers, and kids are less likely to make accurate risk assessments, and this isn't the kind of risk you learn from by fucking around and finding out, you're typically dead. I lived in a place that got very hot in summer with open irrigation canals and we had multiple kids die every year from it. Even adults, but mostly kids.

This is actually a fair point, I think... if the kid has had swimming lessons, it should be fine, but if he hasn't that's indeed reckless to let him play alone next to an open body of water, I'd argue.