From Britax's FAQ -- based on DH needing to go to the emergency room for pain, yes that is considered an injury so it does not meet the criteria for a minor crash and the seats should be replaced.
Britax recommends that the use of a child seat be discontinued if it has been in a moderate to severe crash. We further advise of the National Highway Traffic Safety Administration (NHTSA) position that it is not necessary to replace a child seat after a minor crash.
A minor crash is one that meets ALL of following criteria:
The vehicle was able to be driven away from the crash site; AND
The vehicle door nearest the child restraint was undamaged; AND
There were no injuries to any of the vehicle occupants; AND
The air bags (if present) did not deploy; AND
There is no visible damage to the child seat.
- See more at:
https://us.britax.com/faqs/after-a-c....ljTY3coi.dpuf