In "Same Song" by Pat Mora what fairy tale is the poem alluding, or referring, to in lines 10 and 21?

Part of the success of this excellent poem lies in the
allusion that it contains to the fairy tale of Snow White and the Seven Dwarves and how
the evil stepmother used to look into her magic mirror every day and ask the fatal
question: "Mirror mirror on the wall who is the fairest of them all?" Thus, the
reference in this poem to the speaker's children looking into the "mirror mirror" is a
reference to this fairy tale and the kind of unhealthy focus on beauty and attaining it
that it indicates. Note how the allusion first appears in the first
stanza:

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...peers into the mirror, mirror on the
wall

frowns at her face, her eyes, her
skin,

not
fair.

The answer to the
unspoke question is obviously found in the "frown" of both the daughter and the son as
they find that they do not measure up to society's expectations or standards of
beauty.