"Mainstream media" is an abstract, disjointed entity at best. For starters, it'd be nice to know whether you're referring to the domestic media (i.e. Japan's) or to one overseas; and
if one overseas then to which specific one. America's? Britain's? Russia's?
If it helps, I recently watched a very good film, Kisaragi, which has to do a little bit with the Junior Idols industry and fanbase. The Academy's reaction to the film was very pleasant:
it received four nominations, including Best Director and Best Screenplay (!!!), and it won the award for Best Film at the Blue Ribbon Awards. I've no idea how prestigious those are, but the Academy is analogous to America's Academy, i.e. their awards are analogous to Oscars. Very prestigious.
So, yeah: I don't think the Japanese public-at-large is necessarily horrified by or disgusted by Junior Idols. If they were, I think it would have been hard for them to have received such a film as Kisaragi so warmly.
Then again, the junior idol in that film is 15 years old, not
five. -_-; So it may make a bit of difference. Perhaps if you were to ask the average Japanese person what he thinks of half of the Junior Idol section's members' avatars and signatures you'd find a very different answer. ^^;
