On the surface, it's a brilliant idea. A Rock Band game that young kids and parents can share.
After I gave it some thought, though, I realized that designing it could be tricky. Here are some potential problems I see... and, afterward, some ideas for how to bring rock! into the land of LEGOs.
Young kids have smaller hands, which means they need smaller controllers. Can Harmonix design instruments that are fun for all ages? Or will they expect customers to buy controllers separately for themselves and for their kids?
Part of the Rock Band thrill for adults is nostalgia; hearing songs we grew up with. Young kids won't have that history. But Harmonix knows better than I do what their current demographics are and how vital that is.
LEGO Rock Ideas
There's never been a better excuse to smash our instruments and destroy the stage! Kids love to smash stuff.
Imagine a LEGO character crowdsurfing when he gets pulled apart -- one torso goes left, one goes right.
Let players design/build their own stages and venues. Introducing creativity to the mix adds replay value (which decreases turnover to used copies, among other benefits).
Let players design their own instruments by combining parts. A guitar could have a headstock, fretboard and body.
Kids are as smart as you let them be. Fill the game with plenty of music history.
By the way, I realize many people buy the LEGO games for themselves and not for kids. But I think they're going for family entertainment.