The poem "Fear" by Gabriela Mistral tells of a woman who is afraid of losing her daughter. She compares her child to a swallow that would fly far away, a princess who would no longer play with her and sleep by her side, and a queen who would be inaccessible to her.
This poem is not supernatural. It's not trying to suggest that the woman's daughter would literally turn into these things. Instead, it uses metaphors to clarify how the woman feels. For instance, the woman talks about "them" doing things to the girl. This does not refer to specific people, but rather to society in general that imposes norms of expected behavior upon people.
The metaphor of the swallow flying far away where the woman would not be able to sleep with her or comb her hair represents the daughter growing up, becoming independent, and going away to live on her own. The metaphors of the princess and the queen represent roles of responsibility the daughter might adopt when she is older that might keep her from spending time with her mother.
In conclusion, the poem brings to light fears that any loving mother might have when she realizes that someday her daughter will no longer be completely dependent upon her but instead grow up and have a life of her own.