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.