Rear-ended... car seat questions?
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.