Gris, how old is he?
Not Gris, but I know his oldest is 7 -- just a few months older than Rose.
The My Weird School series (by Dan Gutman) is really popular with this age group at my library. Jon Scieszka's books are great: you might try the Time Warp Trio series or the Frank Einstein series. Mac Barnett has some good series too, like Mac B., Kid Spy and The Terrible Two. Oh, and there's always Captain Underpants, of course!
Others you might try: Timmy Failure books by Stefan Pastis; Origami Yoda books by Tom Angleberger; Yasmin series by Saadia Faruqi (possibly a little on the young side); Big Foot and Little Foot series by Ellen Potter; Gumazing Gum Girl series by Rhode Montijo; Princess in Black series by Shannon Hale.
Rose has really been enjoying the Puppy Place series by Ellen Miles, the Ivy and Bean books by Annie Barrows, the Jenny and the Cat Club books by Esther Averill, and the Calpurnia Tate: Girl Vet series by Jacqueline Kelly. My mom just gave her Ra the Mighty: Cat Detective (by A.B. Greenfield) for Christmas, and though she hasn't gotten to it yet, it looks good and possibly something C might like. Other recent favorites: Dory Fantasmagory series by Abby Hanlon; According to Humphrey series by Betty Birney; Catstronauts series by Drew Brockington (graphic novels).
More mysteries: Encyclopedia Brown, Nancy Drew, Hardy Boys, Boxcar Children: all those old ones are still good fun. Newer books: the Zach and Zoe Mysteries by Mike Lupica; Hilde Cracks the Case series by Hilde Lysiak.
Glad to hear that C is turning into such a big reader!