Like someone already mentioned, calcium can come from better sources than cow's milk. Yes, you'll want to give your child a nutritional supplement just like we need them if we don't eat right (SAD - Standard American Diet). If you eat right, then no you don't need them, but it definitely couldn't hurt to give it to them.
There are plenty of veggies, yogurt, cheese, etc., you name it that can provide a better source without taxing the digestive system. We use soy milk for my 2yo, my 10yo doesn't drink either kind of milk, but gets it from other sources. My 9yo drinks cow's milk, and I actually have to give him, my cow milk drinker, an extra calcium supplement due to leg pain, etc. He is fine as long as he takes his extra calcium supplement. You can't rely on cow's milk for that one vitamin.
Hope this helps!