Joy picks a memory orb, which represents one of Riley's happy memories:
-- Oh, it's that time in the twisty tree, remember? The hockey team showed up and Mom and Dad were there cheering. Look at her, having fun and laughing. (click)
Inside Out, animation
I would only interpret it as referring to time spent climbing around in the branches of the tree. Even if they don't show that part of the story
At or under would also work but describe a different location.