There is a bit of clutter, in the traditional education space. However, there wasn't anything with a good online presence, so we thought it was a great name.

Not going to be able to buy treehouse.com for awhile though, as they want $1m for it.

I was more angling at whether you're concerned about litigation/C&D from existing players defending their TM. It's a great name, but potentially provides an unwanted complication down the line.

As a non-native speaker I wonder what is the relationship between "treehouse" and learning? Except that treehouses are typically being built by kids and kids usually go to school?

We chose it because people generally have good feelings towards Treehouses. Also, they're a place of wonder and fun. Finally, there wasn't anything already established with that name.