My 7 yo DD (my third child, second of three daughters) currently has on her walls a lot of her own artwork, notes or drawings by her friends, random photos she's snagged from me (I don't print many; there are a lot of holiday cards pilfered around here). One wall still has a baby quilt made by a friend; there are a couple of very old embroidered hangings that were my grandmothers that were passed on. No commercial, purchased artwork -- except for the ELOISE poster (yes, *that* Eloise) which my mom had framed for DD...who is *my* Eloise. She shares a room with 6 yo sister (#4, third daughter), who also has similar stuff up -- except her framed poster thing is a "Jojo's Circus" sheet that has info about the day she was born (Jojo's Circus because #4 is also a Jojo in nickname).
Lots of stuffies, Barbies and other dolls, trinkets and a variety of toys. They save and cherish everything -- from the Valentines they got in kindergarten to the trophy for a book club and everything in between. I think you have a lot of leeway! But probably too young for boy bands, generally speaking (mine know well of them thanks to oldest, DD #1, who is soon to be 11). Yes, to Robin's observation of Disney personalities (due to older sister, my little two are totally, like, into all the tween stuff. Yowza.)
Oh, another thing -- they sometimes have been known to draw on walls and furniture with markers. Not quite decor, but, there you go. Truth is, like I said, they covet things at this age, no rhyme or reason. Make a mask at weekly library story time? HANG IT UP! Color a picture at the dentist's office? HANG IT UP! Get a thank you card from a classmate for birthday gift? HANG IT UP! A fortune from the Chinese restaurant? HANG IT UP!
My kids don't have "decor" as much as they have....well, their own interpretation of what I thought might be decor when we were first starting out!
Lots of room for interpretation, as far as your vision goes, I think!