Hi, Suga. How to make ANY tween feel comfortable is my question! Mine is 10.5, starts middle school next year, and has autism spectrum disorder. Not D, but just horrible social awkwardness. I got a couple books on surviving middle school that give advice on everything from social situations to how to use a combination lock, she won't read them but I am! American Girl series is good if your daughter is so inclined.
You might also look for kids with D sites -- I hear there are great ones out there. Good luck with your girl -- ALL of them are gonna need it!