That was my fear, that she would struggle during class and not enjoy it. I'm hoping she loves a single tank BP/W setup as much as I do!
The biggest challenge and turnoff for kids with regards to a BP/W is the time necessary to get things setup. They need to stand there and try it on, you make adjustments, they try it on again, you make another adjustment...then typically they need to put a wetsuit on and try the rig on again, and once more some fine adjustments need to be made.
While you may be really into getting things set up right with the intent that a well fitting rig will help ensure the dive experience goes well, the reality is that all the trying on and what not really sucks and is a chore for the child.
Save yourself the hassle and get an adjustable harness for her. Since you are purchasing a Sub-Gravity short plate (excellent choice by the way...my wife and eldest daughter have Sub-G short plates), perhaps take a look at their Paragon harness...I would give them a call and see if they can substitute a 1-inch or 1.5-inch adjustable/side release crotch strap for the 2-inch they include in the kit, or just keep the 2-inch and purchase a DGX or Dive Rite adjustable/side release crotch strap, they are relatively inexpensive.
-Z