I've never had this happen. But I do have an idea of something to try since you aren't opposed to cheating. You might want to rename your Save before you try this in case anything goes wrong. You can always play your original Save if needed.
Put on "testingcheats enabled" in the Cheat Window (Press Ctrl Shift C) and after it says Cheats are enabled, type in "CAS.fulleditmode", both without the quotation marks. Go into to CAS before she gets taken away, and age her down to a Toddler. Go back into the game. Work on increasing all her Toddler Skills to the max levels. She will then have the Top Notch Toddler reward trait. Go ahead and Age her back up with a birthday cake. (You might want to turn Aging off in the Game Settings so that her siblings and parents stay the same age while you complete the Toddler Skills.) Then check and see what her grade is, and if aging down and back up may have reset it to a C or higher.
I hope you didn't give up yet, and if not, good luck!