I think it is rude, at least in this case. With someone you are close to, presenting information in a nonjudgmental informative way can be acceptable as long as you drop the issue after. Anything that could be interpreted as "If you don't agree with ___, then you are wrong" should be avoided.
In this case, the OP posted that her kids drink a ton of milk and someone responded that drinking milk is disgusting and wrong. That seems to be a pretty direct attack on her parenting skills as far as I can see.