- crianças = children
- meninos = boys AND children
Example - Meninos de colégio (crianças em idade escolar) [school-going children] Similarly, when you say "the dogs in the kennel", you're not only referring to male dogs, but also female dogs (or bitches, if you prefer :-) It's the same logic, is it not?
It isn't the same logic because in English the word dog is not gender specific. Also, while saying menino in certain contexts can mean children in English boys always means boys. Typically only regional slang situations are some normally gender specific words such as "guys" used to cover boys and girls.
No, you use "much" for unquantifiable items, like water for instance. Boys and children are clearly quantifiable. This in English. In Portuguese, "Quantos meninos", "quantas crianças", etc, but "Quanta água", in singular, though colloquially you might hear "Quantas águas", meaning "Quantas garrafas de água".