In one of the sessons "las ninas" is translated with "the children". While this isn't actually wrong, it is safe to assume that those are only girls, because as soon as one boy would be amongst them it would turn into "los ninos". So I guess it would be better, if "las ninas" would be taught to be the girls, while the boys and the children would translate to "los ninos".
I suppose, except that in English we don't strictly use the term "the girls" if there is a group of all-girl children. We could say "the children" even if they are all girls. So it would be wrong, I think, to learn "las niñas" as only being translatable as "the girls". In translating, you would have to make a decision whether to use "the girls" or "the children" depending on what you think the nuance of the sentence is.
I agree with csi. Also, the translations seem to be used in both directions. When you get a multiple choice translation question in which you are given "the children" and must tick "los niños" and "las niñas", then that's because both are stored as correct translations. If "las niñas" were not recognised as a correct translation of "the children", there is a certain danger that we could get a multiple choice question in which it is offered as an option but rejected as incorrect. That would be terribly confusing.
I am not a native speaker in english, so it might be more sbtle there, but in German, if we refer to a group of children it might be a mixed group or we don't care to make it explicit. When I learned Spanish at school, I learned that this would translate to los niños, even if the group happens to consist of girls only. Las niñas would only be used to make explicit that the group only consists of girls. So if there are multiple choice questions which recognize las niñas as the correct translation of the children, that would essentially be wrong. Also translation pages like dict.leo.org translate the children to los niños, only.