He could easily have gone through, as he would have appeared to have been with adults. Many adults keep hold of the child's passport and boarding card as they fear the child will drop them, or have them picked from their pocket. Any staff would have presumed that the child was with an adult and then been predisposed to not checking the child's [non existant] papers.
(To save starting another thread) Then there is the case of a man at Gatwick who found his way onto an a/c that was being cleaned as he was looking for something that he had left onboard. It seems that this was at a gate but he could gain access.
Here is the
BBC story:
Lost wallet sparks Gatwick scare
An air passenger sparked a security scare at Gatwick airport after boarding a plane to look for a lost wallet, as the aircraft was being cleaned.
Travel company Thomas Cook is calling on airport operator BAA to investigate how the man managed to get on one of its jets at 0110 BST on Tuesday.