K.R.
Why are they all out of sight from any adults? Even in our quiet "safe" neighborhood my 6 1/2 year old is not allowed out of my sight, ever. The one time I let him ride his bike around the block with his friend he fell off and got horrible road rash on his knee, he was screaming on the sidewalk and another mom luckily came out to help him until I realized he was gone too long and went to find him.
I have made my house a haven for kids and while their mom's send them out of the house all day and never wonder where they are, they are at my house. I want it that way so that I can not only keep them safe, but monitor their behavior. Young boys get more and more mischevious the larger their goup is, they egg each other on, it's just their nature.
My house is the one with a slip and slide available at any time, a climber in the back yard, extra scooters and bikes in the garage (with a strict helmet and shoes rule), picnic snacks of fruits and veggies at any time they are hungry, and G rated movies when it's too hot to be out. A couple of weeks ago one of the regular "visitors" shouted out of nowhere "This is the best house EVER!" - and all because we let him come in and watch Land Before Time and eat apple slices!
Keep your kid close to home, and his friends too, go outside in the backyard with them when they want to play. You don't have to be playing *with* them, just take a book and settle yourself in the shade, your presence will be enough to keep them in check, and you can keep one ear open for bad language or troublesome plotting ;)