It’s been fun trying to find books for children that describe or mention orthodontics, as most are focused on general dentistry. Here are a few:
Aliens Don’t Wear Braces (#7 of The Adventures of the Bailey School Kids) by Debbie Dadey and Marcia Thornton Jones (Scholastic1993); probably about a 2nd-grade reader.
Braces for Cori by Christine Florie (Scholastic 2005); a K-1 reader.
Buster Gets Braces by Jane Breskin Zalben (Henry Holt and Company, 1992); 1st-2nd grade reader.
What Does a Dentist Do? by Heather Miller (Enslow Publishers 2006); relates a trip to the dentist for a young girl in braces; 1st-2nd grade reader.