This is a tough one. I have grandkids that are 6 years old and under, (much younger than the one(s) you are talking about. I can tell you that I don't take THEM, because they are "too small" and "too young."A great question to ask yourself (and honestly answer) before taking any passenger is:
Can I effectively perform emergency maneuvers carrying a passenger? You don't really know unless you practice it and try it out.
Personally, I wouldn't ride with a child passenger in a traffic situation. I would for short distances in a more controlled environment such as an empty parking lot or rarely traveled residential area.
Goliath616 makes a good point to re-state. In my case, I refused to take "riders" until "I" was sure that I had reached a point in my riding, that I really "understood" my bike and could handle it well, and its nuances.....................I only had my bike for about 2 months before taking my kids on the bike with me (they were 14 and 16 at the time)
yep, I did the same thing when mom wasn't on the bike with us, when she was, I had to sit between them....dad started taking my brother and I for rides when we were around 3 or 4, we always had helmets that fit us too....both my daughters had/have helmets that fit and I made them where pants and not shorts when the rode with me (along with other safety gear)My dad took me since the age that I could hold on, he would sit me on the tank, I would hold on to the handle bars, I wore a helmet with face shield, granted this was I the early 70's... Heck we didn't even have seat belts or car seats back then. I think sitting on dad's gas tank with a helmet was safer than sliding back and forth in the back seat bench of mom's galaxie 500!