When a cop carries on an off-duty moonlighting job, the same rules of engagement apply.
What you're talking about on the other topic is accomplice liability that arises from the felony-murder rule adopted in some states. Except for the driver, the kids in the car were not accomplices to the attempt to run over the cop, so they have no accomplice liability, and it wouldn't have been forseeable to them that the driver would make an attmpt to run over the cop, so they wouldn't have been eligible for M2. Some jurisdictions use the "agency" theory of liability while others use the proximate cause theory of liability, but I've never heard of a proximate cause jurisdiction that doesn't have an exception of application of the statute when the killing of a co-felon is done by a peace officer. If you know of one, I'd be interested to read the statutes.
In order to explore your theory, the first thing that you have to ascertain is if the jurisdiction has adopted accomplice liability. If so, the next thing to figure out is how to shoehorn these kids into felon accomplices if the jurisdiction has no exception for co-felon death by cop in its accomplice liability statutes.