My design and construction notes are all here in the Crafting a Green Post that I wrote for it, but the tl;dr is that it's the Chambray Dress, size medium, from Handmade Style. It's not something that I'd sew for myself, because I prefer a top with better drape, ideally something stretchy, but it's a good look on Will, and she seems to like it well enough.
I did get Will's buy-in on both the dress pattern and fabric before I sewed it, but it's clear now that she'll always want to wear leggings or bicycle shorts underneath it. And that's how my new project has become finding a good leggings/bicycle shorts pattern!
Do you know of one?