I am a firm believer in Murphy's Law. If something can go wrong, it will go wrong.
It's one thing to parse the CoC and determine that your child is actually eligible to take the last flight of the day. However, that does you no good if the kid ends up somewhere and a gate agent or ticket agent refuses to board the kid based on their reading of the CoC.
As you and the kid are waiting for a supervisor to arrive, possibly agree to your interpretation then override the GA, the plane takes off and your kid is stranded.
If it were my kid, I'd just avoid booking the last flight of the day until he's older.