Ds is 6 and has already chewed through all of;
Astrosaurs
Zac Power
Jack Stalwart
Beast Quest
Selby
Tang Shan Tigers
Secret Seven
Famous Five
Enid Blyton mystery books
Jeremy Strong
Judy Moody
Roald Dahl
Alice in Wonderland/Looking Glass
Winnie the Pooh
Complete original Thomas the Tank Engine
He is now reading Artemis Fowl - which I am not thrilled about as it is for older readers, but I do like the fact that he hasn't finished it in 20 minutes like he does with a Beast Quest.
So what's longer and more complex than Beast Quest, but suitable night time reading for a (little) six year old?