People who find an image like this excessively sentimental have simply lost touch with reality -- have learned to reject reflexively any work which appeals too directly to the heart.
The image is contrived, certainly, in posing its subject between the doll of childhood and the glamorized icon of womanhood, but there is nothing contrived about the artist's subversive intention here. The painting was made for the cover of The Saturday Evening Post, which trafficked (at least in its advertising) in glamorous images of women, like the one that is here filling a beautiful child's head with doubt about her own attractiveness.
Rockwell's "Americana", often seen as a sugar-coated lie, had its sharp side. If this image doesn't make you just a little bit angry, and deeply suspicious of a culture that seduces young women into such self-doubt, then I think you just aren't seeing what's plainly there in front of you.