The biting is perfectly natural in a pup as has been said. It is a means of them expressing themselves when they have few ways of doing so, also it is a form of play and learning that they would do with their litter mates and mum, which is what you have now become. You cant stop them doing it at that age, but can begin to put in place boundries i.e what is 'too far', and correct this with a tap and a 'no'. At this age they also really seek contact and so when you let her out she is probably really excited by you and the kid and so just wants to play play play. They also teathe and so will chew