B.M.
First give your insurance agent a call...ask if they will replace them...that would be your best bet...if there wass that much damage there is good chance the seats sustained damage even if you can't see the damage...if they don't want to buy you new seats insist on a letter of indemnity...it is basically them saying that your seats are fine and that if something were to happen to your children while in the seats they are responsible...they'll give you the $$ for the new seats...I know that Britax Customer service is able to answer questions related to their seats...and for minor accidents it's sometimes okay not to replace but with the car being totalled I wouldn't take the chance. Hopefully no one in your family was hurt!
B.