My children were 3 and 5 years old for their first rides. My bike had a backrest and they held on tight. I have been riding for 34 years now and I have 4 professional training programs under my belt including my instructors. Statistically a motorcycle has a 5 times greater chance of avoiding an accident than a car. 1)they stop better
2) they accelerate greater
3)they maneuver better
not to step on any of the toes of those learned answers above but the nay sayers are full of Ka Ka
my girls had the time of there lives on their first ride and their safety was never compromised.
I had a friend who knew this guy who's best friends cousin was killed in a mini van so I made my wife sell hers and get a motor-cycle. (sound familiar)