Hopefully your child will mellow. I was a lot more combative about it at the beginning, and I gained some tact over time. I totally understand why it’s hard to be tactful about something that is/feels so important.
After I’d been vegan a decade or so, my parents said, “You know, when you started with all this vegan stuff, we didn’t get it. It seemed like some kind of phase. But you’ve really stuck with it. It was important to you and you committed, and we’re proud of you for that.”
These were words I never dreamed I would hear from them. They didn’t go vegan (but at least, when they visit, they’re happy to respect that we keep a vegan home, and they eat anything I cook without complaining), but their words of acceptance really helped me to return the kindness to them.