'I know the limits of my bike and the terrain'. I wonder how many people have thought that before crashing on a trail they have ridden dozens of times.....
You justify riding like this with your child by making it sound like there is no (as in zero) chance of you crashing. Im not sure if this is stupidity or arrogance. If (when) you crash, your child gets ejected from his seat. He doesn't have the grip strength or coordination to hang on or protect himself in a fall. Hopefully his poorly fitting helmet will offer him some protection. Like you said, your kids, your rules, but don't expect people not to call you out with what they might think are questionable parenting decisions.