I don't see why the Jr BM's can't wear the same style. They are floor length therefore appropriate. Many years ago when I was a Jr BM (at 13 yrs) at my aunts wedding she had the dresses made. The hems were all measured 22" from the ground to be the same. It was a good thing I was tall.
I also had to get my Jr. BM dress from DB and they had some pretty dresses there. Fortunately my Jr. BM is tall so I was able to get her a BM gown that matched perfectly with my other BM dresses. Good luck!!