Ultramarina beat me to suggesting The Penderwicks and the Anne series. DD is dyslexic but loved these on audio books. Also check out The Mysterious Benedict Society series - it's about 4 profoundly gifted kids. Another possibility is The Sisters Grimm series. It can be a bit dark but an interesting twist - the descendents of the brothers Grimm find themselves living in a town inhabited by fairy tale characters who are now living in the modern world. It's interesting because the series brings in characters from every story imaginable. The later books get progressively more dark but a very cool concept.
I also endorse The Mixed up Files... fabulous!!!! Be careful with Betsy and Tacey - in one book a baby sister dies. That ended that series for us. I don't remember which book it was though. There is also a baby that dies in one of the later Anne books...
