Speaking only to boys: אתם ילדים
Speaking only to girls: אתן ילדות
Speaking to boys & girls: אתם ילדים
Is there no word of undetermined gender In Hebrew that means children, as in French, Spanish, English etc?
Well, ילדים is such a word. You can't know whether they are boys or girls if you say ילדים, just like children in English.
can I say : אתן ילדות??
Yes, assuming you are speaking only to girls.