I think you've discovered one of the paradoxes of cursing. Why certain words or phrases are considered profanity while others used in the same exact context are not, nobody seems to really be able to explain lol.
But the fact of the matter is that the majority of society does see "hell" and "damn" used as expletives as well as other various words as profanity while other words like "darn", "dang", "shoot", "heck" as perfectly acceptable. What if your daughter starts saying "awww rats!" instead of dang? Same context, again, totally different word. Or "rotten"? How about "poop"? I heard one trend where "holy buckets" was used.
Personally, I would be more concerned about her using words that society sees as profanity, because those are the ones that matter. Not trying to advocate conforming to society (as thats usually a BAD thing for Christians to do), but I am just pointing out that it is society that determines what words constitute profanity and what words do not. Most profane words did not even start off that way. Example, the B word is a female dog. Look what it means now. But as far as I can see, "darn" is more of an emotional adjective. Something disappoints you, you say darn to express disappointment.
As for "yeah, I heard you the first dang time", that comes across to me as disrespectful, even without "dang". Back chat.