I've not tried this but, you might try a non-acetone polish remover AFTER trying a small test spot in an inconspicuous spot inside the sweater first. It also works for removing permanent marker and stamp inks from paneling without hurting it. Somewhere I have a paperback book on household tips for things like this, but I'm not able to find it.
You might also try a cover up solution. If you or someone you know is handy with cross stitch, embroidery (floss or ribbon), or try needle weaving a design to cover it with a handmade design giving her a unique, "one of a kind" piece of clothing.
You can find designs just about anywhere you look to adapt for this. I have a book of stained glass patterns that I use for both embroidery and appique quilting designs. Depending on her age, you can even use pages from coloring books as design patterns. I have my own collection of coloring books for this purpose.
I did that with one of my daughter's sweaters when she was in kindergarten. It was orange marker on an olive green sweater that she stained it with. I've "embelished" other items as she's grown, she's 9 yrs old now. On a commercial, tight "knit" sweater that had a stamped floral design along the hem of the sweater. I just embroidered a larger copy of one of the flowers like a brooch or emblem over the stain. For a looser knit sweater, (like a hand knit) I would think the ribbon embroidery or a weaving would work better.
Good luck.