Hijo vs. niño

In the Spanish language, 'hijo' and 'niño' are two distinct words that refer to two different yet somewhat overlapping concepts. The distinction between them lies in their specific definitions and their usage context.

Hijo

'Hijo' specifically translates to 'son' in English. It is often used to express a direct familial relationship between a parent and their male child.
Mi hijo está en la escuela.
(My son is at school.)
Él es el hijo de María.
(He is Maria's son.)
Tienes un hijo muy inteligente.
(You have a very intelligent son.)

Niño

'Niño', on the other hand, translates to 'boy' in English. This term broadly refers to a young male human, not necessarily indicating a familial relationship.
El niño está jugando en el parque.
(The boy is playing in the park.)
Ese niño es muy educado.
(That boy is very polite.)
Los niños están aprendiendo español.
(The boys are learning Spanish.)

Summary

'Hijo' and 'niño' are both Spanish terms referring to young males, but 'hijo' indicates a parent-child relationship while 'niño' is a general term for a young boy without specifying any familial ties. Though they can occasionally be used interchangeably - say, when you call someone else's son a 'niño', these words underscore different aspects according to their inherent distinction.