How big are High School Musical and Hannah Montana for Disney (NYSE:DIS)? Huge, going by the family entertainment giant's claim that licensed merchandise will result in retail sales topping the $30 billion mark this year. Disney-branded wares rang up $27 billion in sales at the retail level a year ago.

Miley Cyrus and the stage-happy cast of HSM are ringing up less than 10% of that. However, with Disney pointing to its tween merchandising surging from $400 million last year to $2.7 billion this year, that spurt in reaching kids between the ages of 9 and 12 is accounting for the lion's share in the $3 billion in merchandising improvement this year.

That Disney is cool again with the set that has outgrown Mickey Mouse and Disney's fleet of princesses is old news. However, it's huge for a Disney that has been reaching beyond young families by building cruise ships, adding more thrill rides to its theme parks, and establishing its ESPN brand as the only sports network that matters.

Disney isn't alone.

"Nickelodeon Expands Product Offerings and Debuts New Properties for Kids and Tweens at Licensing 2008 International Show," read yesterday's press release headline out of Nickelodeon parent Viacom (NYSE:VIA).

A merchandising program for its tween-centric iCarly franchise kicks off this month, complete with a show soundtrack, a DVD release, and a clothing line through -- who else? -- Tween Brands (NYSE:TWB).

Investors can't be naive. It's not as if preteens everywhere were hiding under a rock until last year. Disney's surge is coming at the expense of boy bands, major label CD sales, and probably even Mattel's (NYSE:MAT) Barbie. Disney, and to a lesser extent Viacom, have every right to enjoy their current successes, but they can't make the mistake of assuming that they will own this segment forever.

If kids are fickle and teens are fickle, there is no reason to believe the tweens in the middle will be any different. 

The Circle of Foolishness: