Poetry can have many different voices. The narrator of a poem can be different than the author and can describe a character or persona. The narrator can be a part of the poem or the subject of a poem.
In this poem, the speaker speaks in first person about the other characters and the relationships between them all.
The speaker is the mother of two kids, a son and daughter. She is watching her children and their attempts to be "beautiful." The first half of the poem is about the daughter. The mother notices the effort she puts in every morning to look right and the disappointment that she sees. Despite all the time and effort of getting ready, she still doesn't feel beautiful enough.
The poem then continues and addresses the son. He spends his time and energy on working out and getting fit. And yet the speaker says that he, too, frowns when he looks in the mirror.
The speaker is describing this as a mother, which gives the poem a feeling of motherly love, but also of desperation that she feels. As a mother watching her children's disappointment in themselves, we see that she also might be disappointed in herself as a mother.