I'm guessing there's more to the story than is said here. Maybe it's a bad neighborhood. Maybe there's city/city-like traffic. Maybe it's a country-ish road with no sidewalks, and both kids were at risk of being hit by a car. OR, maybe the nine-year-old didn't know enough to hold the three-year-olds hand while walking. (Young kids, including six-year-olds, can have a tendency to suddenly dart out in the road.) Maybe the older kid was "torturing" the little one (as older kids often do when out of view of the parents and/or if they're been required to let a younger sibling join them. Maybe there's a little pond or other water nearby, and they stopped to hang around at it....
Could have been anything. Not all neighborhoods or kids are alike. I'm fairly certain no police would show up if I allowed a nine-year-old to walk with a three-year-old for a few houses on my street, but it's a quiet, residential, street where everyone kind of knows everyone else for the most part.
All that said, a nine-year-old is generally considered too young (by law) to have a three-year-old in his/her care. The "best nine-year-old in the world" isn't capable of knowing what a three-year-old might do.
People who don't adequately understand children are often prone to thinking that other people watch children too closely. The reason for that is they're just unaware of what they don't see, don't know, don't realize and/or don't think will possibly happen. That's not an insult toward anyone who doesn't understand children well enough or toward anyone who thinks, for example, that a nine-year-old should have a three-year-old in her care. (Yes, there IS such a thing as "over-protecting" a child, but a) this isn't it, and b) much of the time what looks to people who aren't big on protecting children like "over-protecting" is actually just protecting them (which is what parents should do). It's not much of a childhood if someone gets killed or seriously injured when hit by a car, or if a kid falls into a nearby pond and disappears, or if a predator gets them both.
People often grow up seriously resenting their own, say, mother for not allowing them to do some outrageous thing they wanted to do.. So, they grow up with a real "thing" about protecting kids and not just letting them do whatever they want to do at whatever age they want to do it. Parents have to be grown-ups and separate from any "thing" they grew up with - and protect their kids within reason.