I don't know how susceptible you are to feeling shamed and humiliated. It doesn't take much for me to feel shamed. For me, in some moods and some contexts it can feel shaming just to be disagreed with. I think that the T probably shouldn't have argued with you. Often you can make a point without belaboring it. She could have just said "the letter is niece's to write" and left it at that. Particularly in the second session where rapport is just being built, I think it's best to err on the side of being gentle. However, this sounds on the whole like a T who is committed to helping girls find their voices and who will help you to be the best parent you can be--if you let her. That's gotta be a good thing.