Hija vs. niña
The Spanish language has different words to refer to a girl, two of the most common being 'hija' and 'niña'. While both could potentially be translated as 'girl', they have different connotations and applications.
Hija
'Hija' specifically refers to a biological or legally adopted daughter. It denotes a family or parental relationship between the speaker and the girl.
Mi hija tiene diez años.
(My daughter is ten years old.)
Ella es la hija de mi hermana.
(She is my sister’s daughter.)
Como padre, estoy muy orgulloso de mi hija.
(As a father, I am very proud of my daughter.)
Niña
'Niña' refers to a female child, regardless of any family relationship with the speaker. It's a general term for 'girl'.
La niña está jugando en el parque.
(The girl is playing in the park.)
Esta es la mochila de la niña.
(This is the girl’s backpack.)
La niña que vive al lado es muy amable.
(The girl who lives next door is very kind.)
Summary
In summary, while 'hija' and 'niña' can both refer to a female child, 'hija' imparts a sense of familial or parental relation which 'niña' does not. 'Hija' is used when speaking of one's own daughter or someone else's in relation to their parents. In contrast, 'niña' is more general and can refer to any young girl without implying any specific familial relationship.