what seat do you have? Some carseat companies require replacing after ANY crash even small ones.
If you were injured (whiplash is injury) the seats need to be replaced. The at-fault driver's insurance should cover it.
Britax follows NHSTA's recommendations for replacement:
Minor crashes are those that meet ALL of the following criteria:
The vehicle was able to be driven away from the crash site;
The vehicle door nearest the safety seat was undamaged;
There were no injuries to any of the vehicle occupants;
The air bags (if present) did not deploy; AND
There is no visible damage to the safety seat
But given that there were injuries, your seats don't meet the criteria and need to be replaced. Contact your insurance; when my seats needed to be replaced, my insurance replaced them and then later recouped the money from the at-fault driver's insurance.
Yeah, I've been rear-ended twice and neither time have I had least bit of trouble getting new seats.