My kids grew up halfway on a plant-based diet (our whole family was vegan for 10 years) and they grew up healthy into adults. Two have stayed true to the vegan ways, one choose to do vegetarian (dairy and eggs), and the other is a whatever is available to eat college kid - who remembers to do plant-based diet when he eventually gets sick to get healthy again.
They are right, in that it is hard to eat only plants, in a social setting. My kids learned how to say 'No, thank you" politely when offered a greasy pepperoni pizza slice, or put up so-called friends who bully them by shoving meat into their lunches (though it did not work as I made their lunches for them). Or my daughter who would ask what the vegetable of the day was at elementary school, and the lunch lady would shove a plate of french fries on her plate and say "here it is", in which my daughter politely gave the plate back and politely reprimand the lady by saying "That is not a vegetable!" - this was years before the salad bar made its way into the high school at least, and has yet to get to the elementary school. . . sigh.
It was hard to come home after taking the kids to all their after school sporting events and have to come up with a meal in a hurry instead of going to the fast food place when it was done: actually I would prepare something in advance (wet burritos were their favorite that I could just pop in the oven for 20 minutes and they would eat after they took their showers), and have juice and trail mix for them to snack on before the events/games/practices.
I had so many comments that I was making my kids sick and unhealthy eating that way. It has been over 10 years, and they grew up strong and healthy. They played sports in school, and my first son grew to be 6'3" and close to 200 lbs - in other words he got big from plants! They are either graduated from college, going to college, and even my youngest one who is a senior in high school is going to college (dual enrollment), so their brains grew up pretty well too.
Of course my main focus in going vegan was to become healthier, and with each step of the way, I felt better and better. So in my experience, I decided that I would 'coach' my kids to look at the stuff they put into their bodies and ask if it is something that will make their bodies strong and healthy? or be toxic to their systems? Sometimes they don't remember and call me up for advice still.