You paid how much for your car and didn't know about the active head restraint system?
seriously, glad you are ok.
I am 99% sure the active head restraint as well as the belt tensioner are one-time use and needs to be re-charged after they have saved your body from more serious damage. I believe they are powered by CO2 cartridges that are fired electronically when so ordered by the crash sensors/computer. The cost to replace the cartridges will be part of your dealer's repair estimate. No worries, mate.
I have no idea really, but the kid's insurance company is probably looking at a 5-7 grand bill. Would be interesting to learn the cost in due course.