First I have to suggest that engaging people in this kind of discussion is not
productive. Some people (for whatever reason) don't like the idea that alternative
life choises are possible. However, I think you should have an understanding of
what healthy eating is and be confident about your knowledge. This will help you
in your quest for eating well and raising healthy kids. I have two suggestions.
First is the American Dietetic Association's position paper on vegetarian diets.
It is supportive and discusses the ins and outs of eating health.

http://www.eatright.org/cps/rde/xchg/ada/hs.xsl/advocacy_933_ENU_HTML.htm

The artical has several references. One book I also like to refer to is:

The Nutrition Desk Reference by Robert H. Garrison, Jr and Elizabeth
Somer. Published by Keats Publishing, New Canaan Connecticut.
ISBN 0-87983-488-9

This book is not oriented toward veggie living, but can supply tons of information.

When I first became a vegetarian I tried to tell other people (some close friends)
about how easy and great it was. I met a lot of resistance and soon discovered that
I wasn't about to convience anybody about anything. I again have to reiterate the idea
that engaging other people in discussions about diet is not productive. The best I have
been able to do is just be a quiet example of how someone can succeed at going veggie.

I agree that  it's best not to engage in an argument, but a healthy discussion can be quite eye opening - it's too much to post so I'll direct you here http://www.petakids.com/health.html

a few highlights:

Nutritionists and physicians have learned that plant products are good sources of protein, iron, calcium, and vitamin D because they can be easily absorbed by the body and don’t contain artery-clogging fat. The American Dietetic Association states, “Well-planned vegan … diets are appropriate for all stages of the life cycle, including during pregnancy, lactation, infancy, childhood, and adolescence.”(27)

                • Protein—Consumption of too much protein is a much bigger issue than not getting enough. Protein deficiency is common only in countries suffering from famine. Eating too much protein, however, can lead to cancers of the colon and liver.(28,29) Children can get all the protein that their bodies need from whole grains in the form of oats, brown rice, and pasta; from nuts and seeds, including sunflower seeds, almonds, walnuts, and spreads such as tahini; and legumes, including tofu, lentils, beans, and peanut butter.

                • Iron—Some babies’ intestines bleed after drinking cow’s milk, increasing their risk of developing iron-deficiency anemia since the blood that they’re losing contains iron.(30) Formula-fed babies should be fed a soy-based formula with added iron to minimize the risk of intestinal bleeding. Iron-rich foods such as raisins, almonds, dried apricots, blackstrap molasses, and fortified grain cereals will meet the needs of toddlers and children 12 months and older. Vitamin C helps the body absorb iron, so foods rich in both, such as green, leafy vegetables are particularly valuable.(31)

                • Calcium—Cornbread, broccoli, kale, tofu, dried figs, tahini, great northern beans, and fortified orange juice and soy milk are all excellent sources of calcium. As with iron, vitamin C will help your child’s system absorb calcium efficiently.

                • Vitamin D—Cow’s milk does not naturally contain vitamin D; it’s added later. Vitamin D-enriched soy milk provides this nutrient without the animal fat. A child who spends as little as 10 to 15 minutes three times a week playing in the sunshine, with arms and face exposed, will get sufficient vitamin D because it is synthesized in the skin when the skin is exposed to sunlight.(32)

                • Vitamin B12—Whereas other primates get their necessary vitamin B12 from feces, dirt, unchlorinated water, and insects, any commercially available multivitamin will ensure an adequate amount of the vitamin for your child.(33) Vitamin B12 is also found in fortified soy milk and many cereals.