I think adults in general underestimate children and their capacity to deal with and enjoy this kind of stuff. My 7 and 5 year old boys, for example, are huge fans of the Indiana Jones movies. Rarely do my so-called adult games get more graphically violent than Indiana Jones. So, what's to keep my kids from playing D&D with the assumptions are written already?
If anything, it's the rules structure that's going to make it difficult to capture younger audiences. If a potential player is old enough to handle the rules, he's old enough to handle the themes of D&D as is, in my opinion.
After all, didn't most of us start playing in junior high or even younger, back in the late 70s or early 80s?