It's fine for kids to be vegetarians. Meat has B12 but most people still don't get enough. Everyone, not just vegetarians, should supplement to eliminate the problem. B12 is made by bacteria, not animals so it is easy to find vegetarian pills.
Bread, pasta, cereal, rice, beans, peas, lentil, nuts, seeds, fruits and veggies can easily offer everything that you need except B12. All vegetarians should take a B12 supplement regularly since most people eat meat, dairy AND fortified cereals yet still have a B12 status that is lower than recommended.