There may be some reporting issues with your data. For example how many helmets prevented would've-been-reported head injuries? How many injuries aren't reported? And how many hours on bicycle are there compared to constantly living around so many guns?

A natural experiment may require comparing similar communities with and with helmet laws.

Shootings and stabbings on the other had are difficult to not report. And can happen regardless of proximity to bicycles.

Ultimately let's do both: get helmets for kids and get rid of so many guns.