I think there are 2 problems with the as-shot that stand out to me
1) the background isn't evenly lit, which gives you more PP work to do painting it back to white
2) the lit bit also doesn't come low enough down the backdrop. Ideally, it should be totally white above her head and down to, or below, her knees
The most common reasons for above are - the rear lights aren't powerful enough, that they are positioned too high, and that the umbrella/softboxes aren't big enough. This is compounded by the obvious lack of space between subject & background in this shot (presumably you just don't have much space?)
If you can... move both the subject and rear lights towards you a bit, even a foot will make a difference in evening out the rear lighting. If still not right, reduce the key light's output by 1 f-stop compared to the rear (expose for the key of course) and you should see an improvement
The original exposure was probably 1/2 stop under, and although MM's PP is good as a recovery, the skin tones have suffered a bit - most noticeable in her foot
If you can get it right at the taking, PP is simply a wipe around the front with a soft-edged white brush which takes 10-20 seconds and maintains the skin colour more accurately as you don't change the exposure at all
The face-on posing doesn't normally work too well, but you can get away with it with kids - what's really nice in your shot is the clearly unforced smile & connection to the girl, showing you're relating well to your subject
Having said all that, it could just be you're after a highly processed look to the shot, in which case it worked pretty well... and now ignore all of the above
I hope those comments make sense & help a bit?