Is she willing to try this out? At the age of 12, she has to be on board with it. Jeff raised his daughter on a plant-based diet and from what I understand she was very active and fit.
Maybe if you approach it from the perspective of nourishing her body to support her Taekwondo, she’ll jump into it. The great thing about this way if eating is that it is very satiating, and you can eat a decent volume of food for relatively few calories. If a pint of ice cream is 1000 calories, just think how many bowls of beans and rice you can can have for the same amount - it’s a ton of food.
You could just try it out for a month and see how it goes. PCRM has a podcast, and the host had gastric bypass and eats a plant-based diet. The latest episode is about raising vegan kids. I haven’t listened to it yet, but the info might help you with your decision.
https://www.pcrm.org/podcast