It must depend on the situation surely. Where I used to live there was a green about 150 yards along the road (quiet residential) on our side of the road and a lot of local children (it was a village so most knew each other) would play there. Well, I would let my children then aged 8 and 6 play there and take their 4 year old sister there too as long as they all stayed together.
But we have now moved and are on a main road and there isn't a place nearby where children congregate, except a small stream next to our house but I am worried about them falling in to it, so at the moment, although they are now 9 and 7, they don't alone away from the house.