Hold the phone. You were told that yes, your 12-year old can order off the kids menu at a TS restaurant, and yes, you can pay OOP for that meal while the rest of you use your TS DDP credits, but you will probably be charged at ADULT prices? I don't think you should be OK with that. I'm not even sure how that would work. If your 12 y/o orders something off the kids menu that is not even on the adult menu (mac & cheese, for example), what is the "adult" price going to be? At the very least, if your whole purpose is just to save 3 TS credits for use at a character breakfast, where the most common per-adult cost is $18.99 (before t&t), you certainly wouldn't want to end up paying more than $18.99 each (before t&t) for these three OOP meals you're talking about, and if the bill is going to be more, then use the TS credit and pay for the character breakfast OOP.
Despite the fact that you were told on the phone by Disney Dining that they will handle your request in manner X, just know that the CM at your restaurant may tell you that they will handle it in manner Y, because AFAIK there is no official DDP policy for an "adult" (in Disney terms) wanting to order off the kids menu and pay OOP instead of using an adult TS credit (assuming there are still adult TS credits left in the pool for that reservation). I mean, I don't think Disney can force you to use a DDP credit (or in other words, subtract it from your account without your consent) if you say in advance you want to pay OOP for something. What Disney can do is say (and this is what patsal was told, apparently) that an adult can either use an adult TS credit and order from either or both menus (this is essentially the same as using a TS credit but not ordering everything you're entitled to: adult app, adult entree, adult dessert & NA bev.), or pay OOP but cannot order off the kids menu. If I were you, I would be prepared for this to be the case, and then you'll be able to make an on-the-spot decision as to whether to use the TS credit or pay OOP for what your 12 y/o plans to order. For example, if you are at SciFi for lunch, a hamburger fries and Coke off the adult menu is a little over $14 so you could pay OOP for that and save the TS credit; if you are at Coral Reef for dinner, on the other hand, and your 12 y/o wants any entree other than the vegetable strudel, the tab is going to be over $18.99 so you might as well use the TS credit.
The very best advice of course, which was mentioned already, would be to be very nice to your CM at the restaurant and ask oh so kindly if you could pay OOP for a kids meal for your 12 y/o, and then let the magic happen.
Hope you have a wonderful time!