A tree house for Mia

Mia wants a tree house but she needs a tree...and some wood, some nails and a ladder.
She has it all worked out in her 5-year-old mind.
She is a problem solver.
She has seen a tree house like she wants at her friend's house.
It's cozy and up in the air and she can take cookies up there.
The main obstacle is the absence of a tree.
There used to be a tree in Mia's backyard but it was a mean tree with thorns and a rough scaly trunk.
Her dad cut it down to keep kids from scratching their legs as they attempted to climb it and because it was very old and leaning dangerously in the wind.
It now makes for an interesting stump to jump on.
So Mia is searching for a new tree.
She asked me if she could use our tree, looking at me with hope in her big blue eyes.
We have a cottonwood in the back that has some potential as far as she can understand it.
It's tall. It has lots of branches and it's right here in Grandma and Grandpa's yard.
She knows I would help her.
We went out to look at it.
Hmm. Most of the branches didn't look up to the task.
I told this to Mia.
She dismissed my concerns.
"I know where you keep your wood. We can put it up there and then try it. You could get some nails and a ladder and I could climb up there!" she said.
I sighed.
I asked Marc about it.
He was aghast and he fears for our precious tree.
"Mia might just have to do without a tree house," he said, looking at me pointedly.
Ok.
Fine.
I'll just have one more surveying look. Maybe if we propped a piece of plywood on the lower branch or hung a kind of hammock between two of the stronger ones?
Maybe a double-decker?