If Rock Band 2 doesn't make your kids love music, I don't know if anything can.
The game's Special Edition comes with not only the game disc, but with all the different instrument controllers for the game (though you'll need to buy one more guitar controller to have a full band). Kids can play drums, lead guitar, bass, or sing along to the songs and the better they do, the higher their score goes. The game is compatible with all Rock Band 1 instruments and most Guitar Hero instruments. Just make sure you get the version for your console (PS3 instruments for PS3, Xbox instruments for Xbox).
Players can download new songs on the Playstation Network or Xbox Live. Prices are typically $1.99/song or $5.49/3-song-pack. There are some packs that are larger and cost more. There are some larger packs you can buy on disc and import into your console to play. If you have Rock Band 1, you can pay $5 to import all but 3 of the songs onto your console for play in any Rock Band game.
One fault the game has is promoting inappropriate sexuality of women. A player may customize their character, and it is possible to outfit female characters with some more revealing clothing, which I would probably consider inappropriate (iffy at best) for those under 15, maybe under 14).
All songs (both on-disc and downloadable) have their swear words deleted (not "bleeped"), so unless you already have heard the song and know the swear, your kids may not notice the song is even censored. That being said, there are some songs with drug and sex references. If your kids are younger and want to play Rock Band, they should try Lego Rock Band, which is nearly the same game, but much more kid-centric game. Only specially-marked "family-friendly" downloadable songs may be played on Lego Rock Band.
The game is very addictive and incredibly popular. This is the same developer who invented Guitar Hero 1 and 2 before handing over the reins and creating Rock Band 1 and later, Rock Band 2.
If your kids love this game, check out Rock Band 3, due to come out in holiday 2010, which will feature a "Pro" mode which will let your kids actually learn (mostly) how to play the guitar, drums, or keyboard (yes, the keyboard/piano will be added as a playable instrument). New instrument controllers for the game will be released, including those that are Pro-mode compatible, meaning they are real instruments that can be played without the game.
What other families should know
Too much sex