Kids and Curse Words

Since I am such a patient mom, the other day I told Jordan to get to the homework in front of him and clear the rest of the crap off the table. This made him angry and he told me not to cuss and informed me that I told him crap was a cuss word and not to say it. It could have been a moment of "no, I shouldn't use that word." Because I am the patient mom that I am, I sternly informed him "no it isn't a cuss word and clear the CRAP off the table now." Fast forward a few days and Caleb drops a small metal board on his foot, yells "ow, ow, ow, ow" until I run in the room to see what happened to which he replied "that hurt......damn it."
My reply "Caleb, don't say that!"
His: "ok, but that hurt. Damn it."
OK, my oldest thinks crap is a horribly bad word but the youngest sees nothing wrong with saying damn it?