Niñez translates to "childhood". It's a beautiful and evocative word that captures a specific stage of life, making it more interesting than common nouns.
In the song, the artist sings, "Si te llevas mi niñez, llévate la parte que me sobra a mí" (If you take my childhood, take the part that's left over for me). This line uses "niñez" metaphorically to represent a past self or a period of immaturity that the singer is ready to let go of, adding a layer of depth to the word's meaning.
This song uses Peter Pan as a stand-in for the part of us that resists growing up. The singer describes a strange kind of calm arriving just as that "Peter Pan" side starts to feel less like comfort and more like a threat.
Instead of fighting the change, the singer lets it happen. Losing the carefree, childish piece of themselves turns out to bring peace rather than loss, and by the end they picture Peter Pan flying off with Tinkerbell while they stay behind, finally ready to grow up on their own terms.