There are dangers all around. Even in places that we think are safe, there could be predators. Friendly faces that hide all sorts of evil in disguise. Older kids who think it's fun to introduce inappropriate things to younger kids. I could go on and on.
So for that reason, the biggest rule our kids have, is that they have to stay in our yard, when playing outside. By keeping them in our environment, there is no opportunity for secrets. No opportunities for them to be hurt, or exposed to things, without our knowing about it.
We don't mind other kids playing in our front yard, but then comes rule number two: All kids must stay in the front yard. There is no playing in our garage, sideyard, or backyard. They cannot come inside our house. Even though our kids play with each other, I don't know the parents that well, and frankly, they don't know us. Once again, a friendly face doesn't always mean innocent actions.
In order to keep safety and accountability, all the kids stay out in the open. This has worked well for us in the past, and continues to be a good rule of thumb. I've discussed our rules with other neighborhood parents, and they agree with it. When they send their kids out to play with my kids, they know that they only have to look out their window, and they will know exactly what the kids are doing.
Why take a chance with your kids protection? Evil comes in all forms, even from people that you think are safe. Sometimes my kids gripe at these rules, but it takes a lot of effort to keep out of harm's way.