I tried to put together a list of some... My sister is a 6th grade teacher and she has a problem with her students singing Kesha (Tic Toc) non stop which is so not appropriate! Also, there's a children's music section in the itunes store you can browse.
*Glee! I don't know if there are any Glee fans out there, but I love those cds and they have a wide variety of songs.
*Disney mixes! Radio Disney, Disney Movie Soundtracks (search Disney in the itunes store and you get tons of results!
*High School Musical, Camp Rock
*Black Eyed Peas (Where is the Love, I Gotta Feeling)
*Rihanna (Please Don't Stop the music, Disturbia)
*Lady Gaga (Just Dance)
*Whip My Hair (Willow Smith)
*Leona Lewis (Keep Bleeding Love, Better in Time, Take A Bow, Footprints in the Sand)
*Sean John (Down)
*Spice Girls (Say You'll Be There, Wannabe)
*Shakira
*Sara Bareilles
*Fame soundtrack (the new version)
*Taylor Swift
*Rascal Flatts (Life is a Highway, Love You Outloud)
*Owl City (Fireflies)
*Broadway Music! (Wicked, Chitty Chitty Bang Bang, Billy Elliot,
Mary Poppins, Lion King, Little Mermaid, Hairspray, Annie, Peter Pan)
*One Republic (Apologize, Come Home, All We Are, Dreaming Out Loud)
*OAR (Love and Memories)
*Queen (We Are the Champions)
*Mario Spinetti (Man on the Moon)
*Kelly Clarkson (Breakaway, Since You Been Gone, Where is Your Heart, Listen to Your Heat, My Life Would Suck Without You)
*Pink (So What, Who Knew)
*The Fray (All At Once)
*Jay-Z (Empire State of Mind)
*The Beatles
*Also songs by the Killers, Postal Service, U2, Journey. It's amazing what kids like sometimes!