It expresses sympathy, compassion, and often sorrow for a child who is experiencing some kind of hardship, suffering, or misfortune.
Here are some possible contexts for the phrase:
* A parent expressing concern about their child's illness or injury: "Mi pobre niño, ¿te duele mucho?" (My poor child, does it hurt a lot?)
* A person witnessing a child in distress: "Mi pobre niño, no llores." (My poor child, don't cry.)
* A caregiver offering comfort to a child: "Mi pobre niño, todo va a estar bien." (My poor child, everything will be okay.)
The phrase can also be used more generally to convey sadness or empathy for a child's situation, even if it's not related to a specific hardship.