Is it healthy for children to be a vegan?

It can be. Vegan lifestyle can be very healthy for a child if they are taking care to eat a variety of 8-10 servings of fruits/vegetables daily and getting a variety of whole grains and healthy sources of fats. It is still recommended that vegans supplement with a multivitamin out of a concern for low b vitamins.
Think. Please forgive my frankness. The health benefits of any form of vegetarianism have proved remarkably elusive; proponents cherry-pick obscure journals for their facts. Do you want to force your children to refuse to eat burgers, hot dogs, and ice cream with their peers? When they are grown up, they may hate you because of it. You've thought of this as well -- make the right choice.