@SAFC2012 I thought tickets could be returned?
I assume juniors are exempt from ID checks - well under 14 at least as you cannot attend on your own
I don't see how it cannot happen though unless you make sure any booking that includes a child ticket is not subject to an ID check which sort of makes a mockery of the system if anyone with a child can pass tickets to who they like knowing they will not be picked.
I assume the solution (or what I would do anyway) would be scan the adult ticket first but send the child through and then scan the child ticket and adult goes through on that. I would assume even the thickest steward can see its an adult and a child on an adult and child ticket and the order the go through the turnstile is not important