I have 3 children, a daughter who is 10 and twins, boy and girl who just turned 5. When I initially switched to vegetarianism, my oldest was just about 7. I know some people will disagree but I did not feel it would have been fair of me to "force" ( for lack of a better word) my choices on her. Instead I educated her, talked about why I was no longer eating meat, both the health and ethical reasons, and let her decide for herself. By the time she was 7.5 she had declared herself a vegetarian. Now that I have also dropped the dairy and eggs, she is again asking questions, and I again do my best to educate her. I am pretty sure she will be a hardcore vegan as she gets older. My younger daughter has also decided to be vegetarian, but she is still a bit confused about what is and is not vegetarian food...so again she is learning
with help from big sis and me. My son, he eats mostly vegetarian, as I do not cook meat at all, but when out or at a relatives, he still sometimes chooses meat. I worried about making the choice for them as I didn't want them to rebell but to decide for themselves. If my children had been born after I switched my eating, they would have just been brought up as veggies, but I couldn't take away their (at the time) favorite foods.
That's just my opinion, for what it's worth.
If you are deciding to now raise your children vegan, I would approach it much the same way you would anything...explain why you are making the choices you are. There are also plenty of veggie copies of kid foods...my son probably does not realize that his favorite chik nuggets are vegan copies made of soy