Question on moving darts for the Bistro dress

I have a 13 yo daughter who I'm sewing the Bistro dress for. She's a size 0, but the darts on the dress are not lining up for her at all (though her measurements did say that was her size). She's a bit concave--how can I reset the darts for her to fit her better? I've been looking all over youtube for tutorials but I'm stumped! Thanks!