I think the OP was talking about Via Napoli. Via Napoli is a TS restaurant, not CS. So they would want to use three TS credits to buy an extra adult pizza for the child (although it appears the parents wish to eat the majority of that pizza themselves). This really isn't the reason WDW enacted that per transaction rule as all of you are on the same room reservation but the kid just isn't eligible for DDP credits yet, so you can always ask if you can do it. It doesn't sound like they want to buy that pizza and give it to someone who isn't on their dining plan.
Then I think it will be up to the server as to what they want to do.