Hijo can be son or child, but I've only seen hija to mean daughter.
niña would most likely be better for child, and then only if he has 1 daughter and no other children
We often use son, boy, and child interchangeably in English. Then we forget that they don't quite work that way in Spanish.
Child should be either "niña" or "niño". Hijo and hija are for sons and daughters strictly.
It wasn't supposed to be a gotcha. I hope my post didn't come off as smug.