"They don't eat cookies."
Translation:הן לא אוכלות עוגיות.
What or who is the object of the action? Here it's eating, whose eating? Plural females, הן. So you'd need plural feminine. If it was mixed, this female thing & that male thing or if it's plural male, even if it's written like female (אירה, יין; wine, Lion) then you need male plural.