I live in a very safe neighborhood myself where one can walk to the playground without ever crossing a street. Many parents let their kids out to play around 8 or so, but the kids are usually so busy you never see them except holidays. I let my kids go out to play by themselves about that age, they really weren't interested when they were younger. I was certainly letting my son walk to the bus stop (3+ blocks) away on his own by 3rd grade. Now that my son is 15 he bikes 2 miles to school and back on his own.
Not sure what you can say to other parents, as parents have gotten a lot more paranoid about this issue over the past 30 years. If they are older perhaps having them remember what they did when they were kids. I walked by myself to school in 1st grade, 3/4 of a mile of suburban streets.
