@Ace McDunk
There already IS one. What part of the "15" or "18" ratings on the side of games are people missing? It's the exact same damn symbol used for films and DVDs. It even says on the back that it's a BBFC rating.
A conscientious parent shouldn't NEED educating about these things - he/she should be on the lookout for such rating systems; should research the games he/she is buying, to judge their suitability. Be pro-active about supervising their children. At this point it is simply impossible for me to believe that parents are not aware that some games contain material which may not be suitable for young children. Which leads me to the inevitable conclusion that either they just don't care, or, more positively, that they have more faith in their children's ability to separate fantasy from reality than the hysterical lobyists.
Bottom line, this will not change a thing, and I remain to be convinced that there is even a problem.