Parents need to know that some of the outfits show a bit of flesh, and some songs reference adult themes (such as drinking and one-night stands), but for the most part KARAOKE REVOLUTION is good, clean fun. Group play allows players to anonymously judge each other, which may be a bit too mean-spirited for younger kids. Also, even when your kid is playing alone, the audience will turn down the house lights if the performance isn't up to par -- sensitive kids could easily feel hurt. You may want to adjust the judging levels.
It's a lot of fun when you have a group of people to play with. Not so much by yourself. It's too easy to score points, though, because you don't have to be on key, you just have to have the right pitch.
I am an elementary general music teacher, teaching grades 1-5. I brought in my PS2 to try out KR with my 4th and 5th graders, since I was having trouble getting them to sing for me. When they got a chance to don the headset or grab the handheld mic (which now comes with the latest version of KR), all inhibitions were gone. I could get a general idea of where that kid's voice really was. Of course, my whole curriculum isn't based around KR, but it is definitely supplemented by the help of this "way cool" video game. Even my boys run to music to see if it's KR Day. That alone is good enough for me.