My kids (13yo) spends her holidays out in the country with my parents. She’s great mates with the local kids (depending on who’s around there could be up to a half dozen from age 6 to her age) and they build tree huts constantly. They usually have two or three on the go at any time. One of the kids has access to scrap lumber & iron sheeting.
My dad advises on building and safety, but tries to leave them to it as much as possible.
Excellent!
Yep, there were about 5 or 6 of us boys in the neighborhood who would regularly meet up after school and on weekends when we were around 8 to 12 years old. We built our treehouses at a few locations. One was in my parent’s backyard, hidden away in a place where my parents rarely went, so we didn’t have to worry about construction quality 
That particular place we built so high in the branches that it would sway in the wind with even a gentle breeze.
Across the road, at another kid’s place we had a more solidly constructed unit, using his dad’s tools and lumber. That place had three floors built up in the trees against the back fence and a hidden (not very well) basement. When we finished that we had a grand celebration with ‘lolly water’ we’d saved up to buy.
It was still a heaving piece of crap But we had so much fun there - secret codes, walkie talkies, firework wars. Freakin awesome.
As it happens, I’m building one now for my daughter. Unfortunately, the tree is more than a little inadequate, which is kind of what happens when you live in the city. But it’ll be okay when it’s finished. There just won’t be any room for a jacuzzi.
I had the tallest tree on my street, so we built the most ridiculously rickety treehouse I’ve ever seen. It was about six “stories” tall- of course, most of the stories were just levels you had to crawl through to get to the next level. I spent a lot of time up there 'til we had to tear it down for an incoming hurricane. Looking back, I’m quite surprised it held up as long as it did.
“You damn kids! Get inside and watch TV! Yarrghhh!!!”
Something like that?