I have a slender 9 year old. She needs a 7/8 in waist sizes but is more of a 10 in length. I ordered  a 7 hoping it would be short enough not to show under her dress. It worked out well. I'm almost willing to say this runs slightly big. The only thing that's not idea is that it's cut high in the back so it shows over the back of her dress. Not the end of the world though.

She really likes that the top is cotton so it's not itchy.