If you are saying what I think you are saying, I think someone gave you bad info. The child has to be a child on the first day the tickets were used, not bought. Otherwise everyone would buy them for their children and put them away til they are adults.
If you mean you bought them and the first time used, your child will be a child, then yes, at this time, they are grandfathering that child in to an adult ticket. We had to have our grand with us, so they could ask him his age, they swiped the ticket and saw that he was a child when we started. It wasn't really easy, because they had to give us a ticket for that day in case we hopped (we had already just brought him through the turnstile and went to guest services), and then the remaining days, were given as one day adult pass hoppers, that expired..but not til 2025 or something like that. It's darn nice of them to do, and they could decide not to at any time. Oh, and if you have plusses on the old ones..you lose the pluses