Must be See You Later, Gladiator, right? I don't think this book is so bad for third grade. It isn't high-level reading, but I have read the series and would say they're a bit more sophisticated than Magic Treehouse in approach and vocab. It's not vastly far off of your child's tested reading level if that is in the 700s. My opinions may be unpopular, but I'm not sure I'd fight this one to the death if you see other language-based weaknesses. Now, if there are really kids in her group who can't read that's anothe rmatter, but if they literally cannot read they shouldn't be in a group with this book, which is on or slightly above grade level for third. (As I say, I remember the series and it's sophisticated in a way that MTH is not.)
