Well first of all you have to make sure she is in fact lactose intolerant. This will require a breath hydrogen test at any local hospital. If in fact she is lactose intolerant there are over the counter pills but I am lactose intolerant and they dont work for me at all.

My son developed sensitivity to milk about that age also. They would always get milk breaks at school and I started getting calls from the school saying he had an upset stomach. I questioned him and he said his stomach hurt and he felt sick after their milk break. So I started trying lactose free milk at home and told the school to substitute juice for milk when they went for their break and the stomach aches stopped. Every once in a while the school workers would forget to give him his juice and he would drink the milk and he would again have a stomach ache. We never put him on any medications. Just cutting out milk with lactose seemed to help alot.

The best milk i've found is called dairy ease and you can get it in the supermarket (at least here in texas) and it comes in 100% fat free/100% lactose free. I developed lactose intollerance as an adult and used the same milk. To me, it tastes like whole milk and a whole lot better tasting than skim milk and dairy ease is white like whole milk, not the bluish color of skim. Try substituting the milk and see if your child's stomach aches go away. If not, they may be more severe or there may be another cause. Also my son, who is 19 now, has outgrown the problem and can drink any kind of milk now. Good luck.

My son was diagnosed with lactose intolerance in first grade. Cut out all dairy products, not just milk. Ice cream, cheese, milk chocolate, etc., can all cause stomach upsets and pain. You can get lactase in pill form at most general health/ nutrition centers . If they take these with the dairy product, they can usually handle it ok. It can get them through birthday and pizza parties. Educate your child on what they cannot eat. Substitute yogurt products for dairy products in order to insure proper calcium intake. Your child is young enough to learn to like them, if they don't already, especially frozen yogurt.
Lactose intolerance is usually the result of one's body not producing enough lactase to break down the chemical sugar in dairy products. Incidentally, onions and mushrooms contain sugars that are very similar to dairy product sugars, so they, too, might cause stomach upsets in your child.