According to studies and other sources of information...

Children should not drink milk at all for the following reasons:

• Milk often contains estrogenic hormones that are normally given to animals to make them grow bigger and produce more.
• Milk often contains pesticides that cows are exposed to in the fields.
• Milk, when pasteurized, has it's calcium changed in such a way that it becomes (at least) partly unusable to humans and can contribute to bone spurs and other calcium-related diseases. Calcium should be obtained from dark-green leafy vegetables.

There are milk product substitutes such as NSP's famous:Tofu-Moo
(although some are moving away from tofu product because of concerns about aluminum.)

Additional details can be obtained here:
http://health.groups.yahoo.com/group/notmilk/