My response probably falls into the do-as-I-say, not-as-I-do (did) catergory.
For starters, as a new inexperienced bike rider I should not have carried ANY pillon passengers until I had accumulated several thousand miles experience over the first year. The fact that no passengers were injured while riding with me during my short riding career is as much due to good luck as skill.
Four of my grandchildren rode with me during the short 6 week period between buying my bike and the accident (w/o a passenger) which has sidelined me for the past 2 1/2 years. At the time they were ages 11, 9, 7 and 5. The older 3 were fine for short trips around the neighborhood or across town a few miles, and even switching off with each other from the van, every 30 miles on a 100 mile trip to visit cousins. The youngest, a 5 year old girl, was ok for a mile or two at slow speed around the neighborhood. However on her turn at the end of our little road trip, I could feel her grip slip as she fell asleep after less than 5 minutes, and I had to carefully stop and put her back in the van following me with mom, grandma and the other kids.
We decided that she had fallen asleep so quickly because she was so short and had her head turned to the side and could only see the grass in the ditch rushing by. She had been excited for her turn to ride, but I think she was kind of hypnotized by the blur, not being able to see over my shoulder to the front.
I also carried 4 adult passengers on the Vulcan, and was overloaded with every one. Not smart, and I won`t do it again.
So my thoughts now are that 10 or 11 years old is probably as young as I will carry them on a longer or higher speed ride. If I lived where there was more high speed traffic, and multi-lane superslabs I would tend to agree with KM and raise my comfort level to 16 or 17 year old passengers.
Maybe a 5 or 6 year old is ok for short 5 minute trips close to home.
What`s the answer? If you want to take small kids on a bike,- get a sidecar rig.