The main problem with bringing up kids as vegetarians is the difficulty in finding bioavailable iron and B12 in vegetarian food. Some veggie foods are rich in iron (such as pulses and leafy vegetables), but it's a form known as non-heme iron, of which we can process and use about 10%. Heme-iron, which is the prevalent in animal foods, can be processed and used by humans up to 80% (and it is more abundant in animal foods to start with). Pregnant women who are vegetarian NEED iron supplements, I still have not met a single one who is not iron-deficient. And most non-pregnant women could benefit from iron supplements. One of my friends, a veggie, thought she wouldn't need them. She also went through periods of being really tired and distracted. She had a blood test and yup, very low ferritin (hamoglobin is the "currently available" iron, ferritin are iron stores, in lay terms). She started on iron supplements and in her own words, "her life has changed".
B12 is completely absent in non-animal products. Don't let anyone fool you telling you spirulina has loads of it... it does, but it's a form with no bioavailability at all for humans (i.e. you will pee it all out). There is no B12 in Vegemite, and the one in Marmite is added as a supplement (no B12 in yeasts). It is not such a big deal for adults, but it IS for babies and toddlers, and for pregnant women, as B12 is crucial in brain development (and children's brains are not completely developed until 3 years of age).
Spirulina, however, has a very good protein content, very similar to the amino acid composition in meat. So it is recommended in order for vegetarian children to achieve proper muscle development.