Quite honestly, I can't see what the problem is - Child or Adult, either way he is quite entltiled to his seat and the thought of giving it up for a "Fare Paying adult" should never enter the equation!
An infant is not entitled to a seat, however. Sometimes, as a courtesy if we have a quiet flight, we will "block" a seat for use by the INF - but some passengers choose to buy a child ticket in order to make certain they have the seat. This is not considered fraudulent, and neither is what you're doing You Want It When? If we sell an Adult fare at lower cost than a Child (or, for that matter, a return ticket cheaper than a one way) then it is our misfortune if you have discovered it!
Good Luck to you!
