I don't supervise my kids when they play outside (they're 5 and 3). I'll look out the door and check on them every now and then, but other than that they play on their own.
We live in a very rural area and our house is so far from the road that you can't even see it on the main road. We have about 5 open acres they can play on and 4 more that is woods.
If I can't see them when I check on them, I'll yell for them and they'll answer back from wherever they are.