Hi! Since no one has replied to you, I am no expert but I will try my best!
First off, I think that you are smart for getting her cornrows, because any loose style of braids (like box braids, twists, or micros) are too tempting to move around/put up/play around with, all of which can overly stress a fragile hair line. So if you do cornrow it, then make sure she knows not to pull then every which way into tight ponytails, etc... and braiding down the side instead of back and GENTLY is a wise idea...
If it were me, I would also encourage her to MOISTURIZE and to wrap them EVERY night so they stay neat and can be worn for as long as possible (at least a month) to give her hair a chance to grow (and so that her hair is not breaking from dryness under the braids).
Another option that you might want to consider would be to get the girl's hair twisted to the scalp in the front and loose in the back, a style that she could keep for 2 weeks or so and then have redone -- this would be beneficial because:
1. the twists aren't as tight as the braids, so even if they went back they wouldn't be as stressful to her hairline,
2. she wouldn't be messing with it/brushing it to cause further breakage to her front and sides, and leaving it idle and moisturized would give it time to grow, and
3. Leaving the back dorn and out would still allow her to play in her hair -- i.e. put it all up, leave some down, curl it, a side bun, etc... that way, maybe she won't get bored with it and will keep it longer, all without stressing her front and sides! (If she is old enough, you may even want to cornrow the back and have tracks sewn in, providing that she takes care of the hair under the tracks)
I hope you find a style that both of you and her hair can love!!! Young black girls wrap their self esteem up so much into their hair, and I want her to feel beautiful both inside and out. I'm glad that she has not one but TWO mothers looking out for her!