Do you have a reference for this? I scanned TC's website and I couldn't find anything.
http://www.tc.gc.ca/roadsafety/tp/tp13511/tips.htm
Scroll down to used car seats and there's info in a the green box about not buying or reusing a restraint that's been in a collision.
Was there any paint transfer or scratch to either vehicle? If it's so minor that there wasn't even paint transferred, I definitely wouldn't replace the britax seat. Britax has an official recommendation about replacing seats involved in a crash, and minor crashes they don't require a seat be replaced. Infant carrier manufacturers on the other hand, do say to replace after any collision.
My 2 cents - and note that this is a personal story and not a professional recommendation per se: We had a car bump our car when it was pulling out of a parking spot next to us when dd was a baby and in her infant carrier. It scratched the paint on our car, but that's pretty much it. We never replaced dd's seat even though our car cost around $600 for the other person's insurance to fix. There weren't any crash forces to speak of - not like if you were in a collision anyways - a parking lot bump when one vehicle is stopped and another grazes it is probably much more common than most realize... we likely wouldn't have noticed the scratch if dh hadn't been in the car when it happened. Although we had a claim and it was considered a collision because the other vehicle was in motion, it's not something that I felt the need to replace the car seat at the time.
I wasn't a tech back then... but my thinking was that it wasn't really a collision. And while now I'd probably replace the seat just to get a newer one if the other guys insurance was paying for it, I probably still wouldn't replace it if I was at fault and would be paying for it out of my pocket... in my case anyways, it still falls into the category of parking lot ding and not a crash.
So anyways, based on your description, it depends on your comfort level, but it definitely falls into britax's guidelines about minor crash & according to those it's safe to continue using the seat. As for the infant seat, the manufacturer will tell you to replace it, and since you weren't at fault, you might as well replace it. You could make the parental decision to continue using it as I'm guessing lots of folks have hit the curb harder than you were hit from the sounds of it... but since it's not money out of your pocket, you might as well replace it and be done with it.