I'm assuming so. Word for word translations don't always work, and not all words in one language have a word in another language. In English intelligent is synonymous with smart although it is a stronger word, so inteligente is probobly the closest translation. I don't know for sure so don't take my word as scripture, but that's how I understand it.
Yes, they are often taken as synonyms, but they are not exactly the same:
Smartness: can be acquired, not measurable, defined by the ability to adapt, read a situation and apply the information owned up to that point, Practical.
Intelligence: is innate, measurable by IQ tests (in theory), not necessarily pratical.
Listo (ser listo)= Smart. (with the meaning of cunning, resourceful)
Intelligente = Intelligent.
I think that "hijo" is son, and ""niño" is a child (male or unspecified), so your son can be "un niño", but if he is older, if he's 20 for instance, he isn't a "child" (niño) anymore. But he is your child (not translated with "niño" but with "hijo").
Same for someone else's child. These 2 words are not related, a child can be your son, or anyone's son, and your son, or someone else's son can be a child, or not.
In English "children" is ambiguous, as we don't know if they are sons and daughters, or people of young age (kids).
Hijo is used for a son, not a child (if you mean the age), so it's better to say "son", and not "child".
Hijo is a son (but it can also be used in some expression, for instance sayings, to mean an offspring of unspecified gender)
Hija is a daughter.
Hijos means sons or the children of someone (can be yours too), of mixed genders (hijos + hijas = hijos), or of unspecified gender.
Hijas = daughters.
So, the best translation for "hijo", "hija", etc... is "son", "daughter", as you really mention it's the son/daughter of someone, and not any child.
As, when you say "children", you don't know if you mean young people (kids) or children of someone.
But, when you have "hijos", you can translate it with "sons" or "children" (= if they are of mixed gender, or unspecified gender).
un niño = a male child (but not son), a little boy.
una niña = a female child (but not daughter), a little girl.
niños = children, male or mixed or children in general (unspecified gender).
niñas = female children, little girls.
Looking it up sounds like it came from "mi hijo", but the way I hear it used is a term of endearment like baby. Like I worked with an older woman before that called people mijo/mija. It doesn't literally mean baby but it seems to be used in the same context baby is in English, including when not directly referencing a literal baby
It's pronounced like una jota (= the Spanish j).
("Gen" is the stressed syllabe)