Now you've raised a question that will undoubtedly bring a wide variety of answers, accusations, and moral felony convictions without a trial. The official answer is that if you're ten, then you're an adult and would pay the applicable price. The unofficial answer matches your last question.
Regardless of which way you go, be careful if you are making ADRs on the
MDE app just to show yourself and however many "guests" you might have, rather than identifying everyone by name, which necessarily then identifies them by age, too. Once you do that, then it's much more difficult to then try to explain to the hostess why your reservation says four adults but you're telling them two adults and two kids (not that I've ever done that, of course...I've just heard of this happening...a friend of my cousin's mother-in-law's brother's classmate once did it).
We have two girls. Our older one is now 12 but has always been very petite for her age, and when she turned ten, she could easily pass for seven or eight. I will hereby confess and submit myself to the judgment of DISers across the world by saying that we claimed her as a child for a year or two after she turned ten. She ate like a bird, and was never going to eat enough to justify an adult price. Now that she's 12 and has sprouted up a bit, we treat her accordingly and just suck it up for the adult price. On the flip side, I have seen or heard legend of young boys and girls alike who could eat their weight in buffet food at age ten (in fact, the last time I discussed this issue, one proud grandmother boasted that her six-year old grandson could eat an entire large pizza and ask for more...it was a very touching moment). In those cases, I guess it makes sense.
I have a great struggle with the fact that ten-year olds are "adults" at WDW, especially for those meals where alcohol is included (e.g., Hoop De Doo, Luau, etc.). I'll pay a higher price due to age, but when you're asking my tween-age daughter to pay the same price as some thirty-year old ex-linebacker who plans to down three racks of ribs and half a dozen beers in one meal, that's where I have a problem...and I guess that's why we'll be skipping those meals for the next several years. (DISCLAIMER: This post means no offense to ex-linebackers...I'm sure they are very nice people.)