Él es un niño.
Niño is boy. Then why is Él es un niño he is a child and not he is a boy?
Then why cant nina be used instead of nino? nino and nina means boy and girl and both of them are children...
Because the original example used the masculine pronoun, él, which tells us we're dealing with an individual male. As the usage notes in the link I provided explain, the word niño is used to refer to a young boy or a child of unspecified gender. The plural form, niños, refers to a group of boys, to a group of children of unspecified gender or to a group of children of mixed genders. Niña is used to refer to a girl or female child only, and niñas is used only for groups composed exclusively of female children. Modifying the original example, if we said "ella es una niña" it would mean either "she is a child" or "she is a girl" depending upon the context or the intent of the speaker.
You can't use niña because you already have él, which tells you that you're talking about a boy. Also, in Spanish, the masculine form (like niño) is used for either the case where you know you are talking about a male person ("boy") or when the gender of the person is unknown/unspecified (or, with plurals, if it's a mixed group). So niño can mean "boy" or "child [gender unspecified]," but niña is only used to mean "girl."
Niño is the masculine gender-specific word for a male child. Niña is the feminine equivalent. There is no gender-neutral word for "child" in Spanish, so niño and niña are both accepted.