If anyone's head hit the window or side of the car hard enough you can die from an impact speed as slow as 5mph. Due to stopping so quickly the brain plows through its protective layers and smacks into the skull. It gets bruised. No biggie, right? Everyone seems fine. Then the brain starts swelling in response to the bruise (to heal itself) which then pushes more of the brain against the skull. If that's hard enough it starts damaging other parts which then start swelling and sooner or later that low speed can't-hurt-a-fly impact is causing the person to have to get holes drilled in their skull to relieve the pressure.

Now, with front to back collisions using seatbelts at low speeds it's almost impossible to hit your head on something hard fast enough to cause this level of damage. Plus cars made in the past several years have side impact airbags to prevent damage in the kind of situation you described from happening.

With prompt medical attention once symptoms show up the damage is rarely life threatening, but if you're away from society a low speed impact can be fatal. Obviously a drive through is probably close to some hospital, but the point is don't be so quick to shrug off the damage caused by even very low speeds accidents, especially ones from the side.