Was on a camping trip w/ several other familys when I took my son for his first ride. My wife actually suggested it. He was 2.5 yrs old. My wife got on the back and he sat between us. It was a pretty deserted dirtroad w/ less then 6-7 cars a day driving down it. I took my son then I ended up giving rides to most of the other kids.
It will be a while before I take my son on any real rides, but a few times I've hauled my bike over to a dirtroad area and have taken my wife and son for a spin.
My son has fallen asleep seveal times on the bike. But for now he only sits between the wife and I.