Acs is absolutely right! Captain Underpants. My kids have also loved the Percy Jackson series (but there are lots of words). I highly recommend the Myth-O-Mania series (now 8 books) by Kate McMullan. I just looked them up on Amazon & Barnes/Noble and I can't believe they're out of print! Little paperback books sell used for $33 to >$100! My kids Loved these, they were easy (still 9-12 age group) and fun. See if your library has them or can request.
Also, kids have a hard time resisting Roald Dahl books (reward with the movie "Matilda" after the book--Great movie--ooh needs to be added to the movie list about gifted kids!) My DS7 and I are reading Andrew Clements books now. They're always good.
One thing I found with having young advanced readers was that while they were able to read/comprehend at high level, the high level books may not have held their interest, or may be too long to keep their interest (so by the time they'd finish and AR test, they wouldn't remember a lot of the details). I tried to help them by reading a chapter to them, then they'd read the next chapter, and so on. This would make it go faster (they could test more promptly and not forget). It also gave them incentive to read because I couldn't read the next chapter till they had finished theirs and they could recap theirs to me verbally which helped retain what they had read.
I only stopped reading to them at about age 9 (so I'm savoring my reading time with DS7).
