When building cubby houses for kids, it important to have their involvement.
Too often a perfect cubby house sits unused. This slab hut has a plank for
access, and a flying fox wire cable running through it, so the cubby is the
jump off point. Great fun when the friends come to play.
     

By learning thatching, they are learning the principles of waterproofing.
The apprentices spent the night in their creation. The cubbies they build themselves are used more often. Especially if they can sleep in them overnight. They do not need to be perfect, just safe.Next time they stay in a log cabin, they will use a mosquito net!

Castle made for a TV advertisement.   Very early days while still a university student. An advertising company asked the university industrial design dept, for a designer/builder to build a castle,and they recommended me

Cubby hidden under the pool makes a perfect hide away where the brave can spend the night.

There is no way of predicting how the kids will use a cubby. I didn't exactly think of designing car ramps for this tree house. 
Sometimes their building can best be described as "statically indeterminate". You need to quietly add a bit of reinforcement or bracing.
They never notice.