I started touring with my son when he was eight, on everything from rural roads to NYC. It seemed to work out best when I was behind since I could correct him immediately if he drifted, etc. Plus of course it helped that I was a larger, more visible object for cars coming up behind us.

The only time it was really an issue was when we had a speeding car come within an inch of right-hooking him when we were in a city bike lane.