I, personally, do not believe that we should stop allowing ourselves the joy of children's literature once we reach a certain age, and feel sorry for those 'adults' in the world who do not allow themselves the bliss of a well written children's novel.
I recently finished reading The School For Good and Evil trilogy, a series of children's books which, quite frankly, were fantastic. I also continue to enjoy Derek Landy's Skulduggery Pleasant, Suzanne Collins, Greagor and the Underworld and even, on the very rare occasion, return to my old Felicity Wishes and Angelina Ballerina books.
All of these books are well written, some are beautifully illustrated, and I am not ashamed to read them on the train surrounded by people. In fact I like to brandish my children's books, feeling proud that I can still enjoy their light storylines, laugh at their jokes, be stolen away by their magic and colour while others my age have committed themselves to Trollops largest novel and only now decided to read Jane Austin's works, most of which I read in my teens, because I wanted to.
Although I understand preventing well-read eight-year-olds from picking up the works of Defoe, or diving into the likes of The Time Traveler's Wife, I see no reason why those of an elder age cannot return joyfully to the stories they loved as a child, or simply pick up a well regarded children's novel to read themselves rather than to their child. During the time of Harry Potter, no adult had a problem with reading the children's book series, but, sadly, it would seem that J.K. Rowling's world-changing series was a one off for the British adult.
For me children's fiction is by far more relaxing than any other form of fiction, they are the books which are written for young, tiny people, easy to read with action packed storylines to keep even the most distracted of child interested from cover-to-cover. Making them not just fun to read after a twelve hour shift, and possible to read with blurry eyes, but exciting.
If I were to advise anyone to do anything, it would be to pick up a children's novel, one with a blurb which grasps even your adult attention and to read it. I can assure you that after the joy of returning to youthful reading it will be hard not to want to revisit the children's section of your local book store again.