The 26-Storey Treehouse by Andy Griffiths and Terry Denton

This is Andy and this is Terry. They live in the world's coolest treehouse! They're been doing some building work and now their treehouse has: a skate ramp with a crocodile-pit hazard, a mud-fighting arena and a maze of doom which is so complicated that nobody who has gone in has ever come out again, until now...
 
Suitable for ages 9-11
 
Collect the series