Let your kids go out and ride bikes and you may end up with one getting hit by a car. Those are the risks every parent has to manage.
But if we let the edge cases dictate how we raise our kids, we end up with what we don't want - overly managed bubble-youth kids who can't think for themselves.
Is that a risk or a near certainty?
Looking for online guidelines, I find this:
> According to an American Academy of Pediatrics survey of social workers that was posted in Science Daily, children under 12 years old should not be left alone for more than four hours. The social workers who were surveyed also concluded that child neglect will likely be considered when children are injured during that time period. --- https://www.tedbakerlaw.com/child-unsupervised-neglect/
A scraped knee might be injury enough.