Niños are boys. Muchachos are guys. Just a slight difference in age implied there. Also there's not enough context to use the imperfect form of beber instead of preterite so I wouldn't say "bebían" is accurate.
Same reason why the English sentence doesn't have the water either. Articles in Spanish are a complicated matter, in this case, the verb (beber / tomar) doesn't require its object to use an article.
Yes. The word "el" is for singular, masculine nouns. The word "la" is for singular, feminine nouns. "Los" is the plural of "el" while "las" is the plural of "la".
El niño / el bolígrafo
Los niños / los bolígrafos
La niña / la falda
Las niñas / las faldas