The way sizing works is like this:
A girls medium is usually size 8-10.
Girls large is a 12-14
Girls extra large, also called a preteen, is a 16.
A women's extra small and sometimes a woman's small is the SAME size around as a girls large.
A woman's medium is the same size around as a girls large.
And most of the time a girls pre-teen is the same as a medium or a large in a women's size.
They do this so you can have a little girl look a lot longer than before. A girls pre-teen is going to be smaller in the armholes, not have low cut fronts to show off cleavage, and basically be more young looking.
Where a women's outfit will have a sexier lower cut, larger holes around the arms, it will often have darts or shaping for the larger bustline, and if it's a swimsuit, leo, or underwear it will almost always have the thigh high cut around the groin area.
My granddaughter wears a 7-8 size bottom and can wear a gilrs 8-10 top. She can also fit almost all women's extra small in shorts, tops, and anything that doesn't have a lot of shape to it. She can wear a ladies leo, ladies basic tees, a ladies jacket, etc....the only place it's going to be a bit baggie is if it is supposed to be fitted around the hiney.
Her show team leo is a ladies small and I only took the strap up abut 1/2" and took a tiny seam from the natural waist down towards the hip joint to narrow the bottom a bit.
So look in the ladies shops for her suit. You will find some appropriate things if you look long enough. It might be hard but she could always wear something that is less structured and more her style.
Here's what I got when I googled girls interview suits.
http://www.crownroyaltypageantresale.com/Crown_Royalty_Pa...
http://www.everything4pageants.com/Little-Girl-Interview-...
