Fortunately the Zoo has a nurse on hand. They got Ellie cleaned up, and gave her an ice pack for the rapidly swelling goose egg on her poor little head. Maisie stayed with Ella until Ella calmed down enough to go to the class.
On Sunday we went to the Science Museum, where Ellie actually spent time trying to get another bump on her head, "so they'd match." Clearly no long-term trauma has been suffered.
Now, almost a week later, it's still a little swollen but has faded to a lovely yellow-green color.
It's still not pretty to look at, but it's definitely on the mend. And honestly, the injury is much less severe than it could've been, all things considered. No broken noses or chipped or broken teeth - just a goose egg on her forehead. Still probably took a few years off of Aaron's life though.