If you listen hard enough, you can hear the "s" at the end of "pauvres" because "enfants" starts with a vowel. This link might help you a bit with French liaisons http://french.about.com/library/pronunciation/bl-liaisons.htm
Because kids and children are not equivalent? They are in different registers. Kid = gosse, gamin, môme. Child = enfant. There is a car insurance advert in the UK which uses an elderly American actor advising two young twenty-somethings about insurance. He says something like "You kids should try X". It worked, because he was in an advisory "older and wiser" role. But could he have addressed them as "You children"? It would not have worked.
"You guys" is another common usage. Does it mean a group of men? A mixed group? A group of women? I think that it has become so well established now that it can mean any or all of these. But it is not equivalent to "You men" etc, or even (British) to "You blokes" or the more eldeerly "you chaps" (both definitely meaning men). I suppose you could say "you girls" for a group of women, though for some that is politically incorrect. And it does not make it equivalent to "you women".
Though I do remember a film clip in which a marine sergeant shouts to a barrack room of male recruits, "OK, ladies, grab your gear!. We're outta here!" I guess context is everything...
I wouldn't translate "Pauvres enfants" with "Poor girls". If one refers either to a group exclusively composed by boys or a mixed group of boys and girls "Pauvres enfants" is correct. But if the group is composed exclusively by girls the correct translation would be "Pauvres filles".
I think I found an easy way to remember the definition of "pauvre."
When it comes before the noun, it means the unfortunate kind of poor (e.g. "Those poor children had to wait in the car for their parents."). And, when it comes after the noun, it's in regards to money and being broke (e.g. "Those children's parents are so poor, they can't even afford to give their children decent clothes.").
So, in English, whenever we say someone's the unlucky/unfortunate poor, we put it before the noun, rarely after the noun. The moneyless version can come before, but it comes after more often ("The poor child" vs. "The child is poor"). So it's kind of like how we use the word "poor" in English which makes it easier to remember for me.