The mother is right. You've captured her essence, and gotten a good likeness. There are a few things that could use tweaking.
It looks to me like her nose is off a little. The nostril on our left looks off, but when I compare it to the photo, it's hard to tell what's wrong. It's something subtle.
I'm guilty of drawing a line between lips, but I guess I can only see the problem on other people's work. Look in the photo. The "line" isn't a line at all. It's a shape, that happens to fall on a linear path.
The edge of the face, on our left, could be softened, to indicate that it is turning away from the viewer.
Buy more pastels. I think you need a fuller range of colors to match the darker values in fleshtones. The shading on the neck and nose is a muddy color. Maybe not muddy so much as not soft enough for this sitter. It looks like a color that would work in a man's portrait. In the photo, these shaded areas are a soft, almost light color.
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