Well, after her involuntary high-speed dismount yesterday, and the subsequent medical festivities, the_child and I went out after school (first day, today) and bought a new helmet. The old helmet is consigned with appropriate gratitude to the dustbin of history. The bike checks out ok, so she's back on two wheels today.
She also informs me that when things went south yesterday she tucked and rolled, as she'd been taught in her Do-Jump classes.
Yay for getting back on the horse. Or vaguely horse-like velocipede, as the case may be.