My DS is 6 1/2 and likes reading Diary of a Wimpy Kid and Dinosaur Rescue at the moment. He also enjoys the I Spy books.
DD1 is 8 and her favourites at the moment are also Diary of a Wimpy Kid (she seems to always have one in her hand), Thea Stilton and she has just discovered Pearlie so has been getting them out of the library.
Roald Dahl and Diary of a Wimpy kid are great for both boys and girls.
DD1 also has on her bookshelf: The Australian Girl series, everything Enid Blyton - including The Naughtiest Girl series, and Secret Seven and Famous Five (these were her favourites at age 7), Go Girl series, Bindi books, Billie B Brown (good for 6 year age group).
They both still love Dr Seuss - particularly good for 6 - 7 years, and also into Where's Wally at the moment