Translation:How many boys?
Niñas can represent children if all the children present are girls. However, if all the children are boys, then you would use niños. And if there are a mix of girls and boys then you would use niños. Even if there are all girls but one boy, you would still use niños when talking about the group of kids. It's just how it works :)
In spanish, the masculine form of a verb or noun is more dominant. Therefore, when you have a group of children, (girls and boys) the masculine form (niños) will be used. Although it can be confusing,(and sexist) it will be easier to understand once you see other examples. Hope that helped......