Like "Usted es un niño", you would never say "usted" to a child, which doesn't mean the phrase is wrong, it's perfectly valid. Again, notice the sentence of the excercise, "bebemos la leche", you have to translate, in this case, each word. I'm a native spanish speaker, by the way, so I know what's natural or not to the language.
nosotros is masculine, but can include female when you are talking about all, like "nosotros somos humanos" "we are humans". But when you use nosotras, is always femenine. In this sentence "Nosotras bebemos la leche" we have to assume that what is being talked about is a group of woman (which we belong to), is drinking milk. Nosotras refers to the group, not the milk. If you are in a mixed group or men or women it would be Nosotros, or if the group is only males it would be Nosotros. Hope that helps
Yes, but you cannot omit it if you are referring to specific water.
I drink water - You aren't drinking water now, but you usually drink it
I drink the water - There two types of drink on a table. There is either a glass of orange juice or a glass of water, so you tell yourself "I will drink the water", since "I will drink water" in that case would sound awkward.