My son is almost 6, and we have been enjoying the Magic Treehouse series for the past year. However, I would love to read a longer chapter book with him, and I was hoping for some suggestions. He is fairly advanced and has a great attention span, but I need something without themes that are too mature for his age. I'm not sure if he's quite ready for Harry Potter or the Diary of a Whimpy Kid, but I would love to hear from anyone who has read these or other similar series that they would recommend for this age group. Thanks!