I've heard that getting your children to drink chocolate milk so that they will get some milk somewhat defeats the purpose, as the chocolate prevents the calcium from being absorbed. Is this true?
Official answer: we aren't sure, but probably not.
Here is an article from the Mayo Clinic which says:
The jury is still out, however, on whether chocolate causes problems for healthy people who eat calcium-rich diets. [...] In the meantime, if you get the daily recommended amounts of calcium and vitamin D from food or supplements, and practice weight-bearing exercise, eating chocolate in moderation is unlikely to adversely affect your bone health.