OK lets define this a bit. If you are lucky enough to have a street where your kids can play out, you let them. We mums in our cul de sac would come out with the children from quite young and sit on the walls and watch them. We had a boundary restriction which we explained from the start, but we watched to keep them in it.
Gradually, as they got older they played alone, boundary the same, and watched - often. We made sure they were still there, came in before dark because they were still there to bring in before dark. You are the parent, the protector, its what you do. There is nothing more important that needs doing than being out there or going out there often and checking they are still there and safe.
Children need to have freedom in increasing doses. Of course it would be ridiculous to suddenly let them roam miles after only experiencing their own door step. They would become lost, frightened and in great danger as we have seen.
Eventually, not at 6 - 7, they can go a little further. By the time they go on to secondary school they should be confident, know boundaries because you have always set them, and as safe as you have made them.
Of course it is not fool proof. Accidents will and can happen but let us just stand back and really understand how very young these 2 boys were.