Safety fencing and gating should be a requirement of private insurance companies, as they already are. Children can literally drown in a couple of inches of water. They can drown in a puddle. They can drown in a koi pond. They can drown in a bucket. They can drown, and way more often do, in a bathtub or toilet or sink.
* * *
Children can also:
* stab themselves with scissors, kitchen knives, pens, pencils, nails, screws, knitting needles, sewing needles, safety pins (the worst-named thing ever), straight pins, push pins, forks...
* slip and fall and hit their heads, fall down stairs
* choke on just about anything left nearby that fits through a toilet paper roll tube
* hang themselves in various situations on those swings, bouncy play things, or even just clothing
They can and will get into things without supervision, and even sometimes WITH supervision around. You have to parent. Even then, tragic things will happen, and it's an awful thing for everyone involved. I don't think the parent here is going to use this as a platform for their rights. I think they are going to feel stupid and shitty for quite some time if this story is even remotely close to being true as published.
To get back to laws, there are not gate safety laws that require a gate closing off the stairs. It is a "concern" that can be talked about if you're already in the system, but there are not random gate checks. You are not even asked about it on your homeowner's insurance. It's up to you to know that you should prevent very small-yet-mobile children from tumbling down the stairs. Strangely, babies themselves help out with this because they are awfully careful with stairs

but the point is that it's pretty dangerous stuff, yet we do not cry out for mandating it when a child does manage to fall down the stairs.