I'm treating this as If I'm a bystander, and I'm in MY neighborhood, and it is MY neighbor.... which makes my getting involved a little different than if it were someone I'd never met... but let's not get into the "bystanders should just stand back" discussion!
My first step is to secure the vehicle... that means that I probably get the vehicle into park and get the girl OUT of the drivers seat... if I can chock the wheels with something handy (like a big rock) that wouldn't be a bad idea. I'd probably even do this before or during my call to 911... because the last thing I need is an inexperienced teenage driver trying to "back up and make it better".
So.. the next step is to get 911 on the phone, and get the calvary coming... I'd identify myself to the call taker as an off-duty EMT... and make it VERY clear that I needed FD, probably a 2nd rescue, ALS, BLS, and aeromedical on standby.
Of course, now I've got the sister running around and screaming... which isn't helping matters... she needs to either shut up or go away... but the PD should be onscene in about 4 minutes, so that would be THEIR first job.
Depending on the kid's position, the vehicle needs to be lifted. Many of the driveways in my neighborhood have a slight uphill slope, so that will make things more complicated. If the kid is under the vehicle, FD will have to lift the car with airbags, high-lift jacks, or a combination thereof... after they stabilize the car VERY well.
If I can access the kid's head, I'm going to take manual C-spine stabilization and start TALKING to him, try to calm him down a little, and work on a primary assessment by talking with him, and trying to figure out what hurts.
If this happens during the day on a weekday, the VFD will be short-staffed and there won't be a bazillion people onscene looking for something to do, so I'll likely end up involved in the assessment and packaging of the patient. If it is weekend or in the evening, the vollies will show up in force, so I'll be out of the picture and talking with the PD about what I witnessed.
Assuming I'm still around the patient, once the vehicle isn't preventing their movement, stabilize the patient as best as possible and get the patient out from under the car. Once the pt. is out from under the car, then we do a head to toe assessment for major life threats (severe bleeding, etc) then we have to get the bike and Pt. seperated. If we can do that without cutting the bike, it will probably take less time..but we've got a full set of hurst tools... so if we want to cut the aluminum tubes of the bike, that isn't a problem. While we are doing this, we need to work on the airway... if it is still patent, he gets O2... if it isn't, he gets an adjunct or intubated.
Once we get the Pt. and bike disentangled, we need to get the Pt. fully immobilized to a LSB or Pedi-immobilizer and splint what fractures we can... then run to the LZ or TC.
Given that I am a primary EMT with the local FD, but I'm not active because of volunteer politics, I would probably be pushed out of the way as soon as possible... of course, because I'm a member and primary, I might end up riding it in... depends on lots of interpersonal interactions on the scene and who is (and isn't) around.... I love volunteer FD's.
