A native Urdu speaker here, trying to learn Hindi script. Although I am not grammatically inclined, The हैं in the end has nothing to do with negation. It is sometimes dropped with negation but not always. In fact the plural feminine part that हैं depicts can be incorporated with the verb at the end and can be written like लड़कियां पानी नहीं पीतैं. Please note that ैं part on the end of the original sentence depicting nasal H can also be transferred to the verb DRINK पीतैं.... LaRkiyaN paanee naheeN peeteeN. N = nasal shown with a dot (bindi) on the two accented lines over "t". That will have the same meaning without the हैं .
Yes. Please refer to this link - https://www.duolingo.com/skill/hi/Plurals/tips-and-notes