Glee is a show which many tweens/teens are into at the moment. It is based around a high school glee club (choir) and the challenges that the students and teachers face every day. The humour is subtle and clever, the characters interesting and the singing is really fun.
The fact that the show stars teenagers makes it appeal to children, but parents be warned that there is some sexual content discussed eg. celibacy, teen pregnancy, premature ejaculation and sexting. The language is sometimes a bit salty, and in some episodes there are references to dealing medical marijuana.
The other thing is the diversity of characters shows that differnet people can work as a team. In the glee club it is a mixed bag: there's the jock, the cheerleader, the rebel, the gay, the black one, the asian one etc, but they all get along and support each other.
Some characters may promote bad behaviour, eg Puck and Sue Sylvester, the very mean cheerleading coach, but overall 12 year olds should understand the sexual and moral topics discussed and replicated in the show, and if not then I'm sure they would be comfortable with talking about it with their parents.
Glee is a very funny, very clever, and very musical show which I love and I think will appeal to just about everyone!