I was a precocious reader - as is my 7yo DS1 so I sympathise with the difficulty finding appropriate content when their reading age outstrips their chronological age.
I don't think it is so much about "challenging" as "broadening horizons", introducing books about subjects or time periods they aren't so familiar with. I'd suggest going with classic 20th century children's fiction - good quality writing but less edgy/teenagery content. I'd recommend the following as a good selection of different genres, all great well-written stories without much in the way of sex/drugs/gore (some may be easier to find second hand):
-Rosemary Sutcliff - Eagle of the Ninth, etc. (historical fiction set in Roman Britain)
-Arthur Ransome - Swallows & Amazons series
-Laura Ingalls Wilder - Little House on the Prairie series
-Ronald Welch - Knight Crusader
-Robert Westall - The Machine Gunners, Fathom Five (about British children in WWII), The Scarecrows (a ghost story, very scary but I loved it), The Watch House (ditto).
-Rumer Godden - Thursday's Children, Listen to the Nightingale, The River (she wrote many more but the content gets a bit adult after these)
-Ian Serraillier - The Silver Sword (about refugee children during WWII)
-Esther Hautzig - The Endless Steppe (another WWII story)
-Susan Cooper - The Dark is Rising series (fab magical fiction - JK Rowling eat your heart out)
-Mildred D Taylor - Roll of Thunder Hear my Cry (about a black family living in the southern US in the 1930s)
-Nicholas Fisk - Space Hostages, Trillions (science fiction, a bit dated now but a great writer)
-John Wyndham - Chocky (gentle story about an alien intelligence befriending a young boy)
-Susan Coolidge - What Katy Did, etc.
-Gene Kemp - The Turbulent Term of Tyke Tyler (great school story - don't peek at the ending!)
-Jean Ure - not the bubblegum stuff she seems to have sold her soul for more recently, but older books like Hi There Supermouse, Nicola Mimosa, A Proper Little Nooryeff (all about ballet, there are boyfriends but no sex)
-Robin Klein - Half Way Across the Galaxy and Turn Left, Hating Alison Ashley (an Australian author so not sure if available in the UK but very funny)
-Lucy M Boston - Children of Green Knowe
-Philippa Pearce - Tom's Midnight Garden
-Mary Norton - The Borrowers, Bedknobs and Broomsticks
-E.B. White - Charlotte's Web
-Anna Sewell - Black Beauty
-Judith Kerr - When Hitler Stole Pink Rabbit (another WWII refugee story)
-Norton Juster - The Phantom Tollbooth
-C.S. Lewis - Narnia series
-J.R.R. Tolkien - The Hobbit
-P.L. Travers - Mary Poppins
-Elizabeth Beresford - The Wombles
-Lynne Reid Banks - The Indian in the Cupboard
-Michael Ende - The Never-Ending Story
-Simon French - All We Know (lovely understated story about growing up/leaving childhood, another Australian author)
-Bette Green - Summer of My German Soldier (about a young American Jewish girl who befriends a German prisoner-of-war)
-Louise Fitzhugh - Harriet the Spy
-Jan Mark - Hairs in the Palm of the Hand (2 wickedly funny short stories set in schools), Thunder and Lightnings
-Last but not least, the incomparable Antonia Forest - she wrote a series of stories about one family set at school and during the holidays, the "school" ones are hard to find but the "holiday" ones have been reprinted. She also wrote 2 historical novels (The Player's Boy, The Players and the Rebels) set in the Elizabethan theatre world, which are a fantastic introduction to Shakespeare.