why asking billing information when i download free apps.store

- Add a valid payment method to your existing account.

or

- Create a NEW account/ID for her using these instructions. Make sure you follow the instructions. Many do not and if you do not you will not get the None option. You must use an email address that you have not used with Apple before. Make sure you specify a birthdate that results in being at least 13 years old

Creating an iTunes Store, App Store, iBookstore, and Mac App Store account without a credit card

More details of how:

http://ipadhelp.com/ipad-help-tips-tricks/how-to-get-free-apps-from-the-app-stor e-without-a-credit-card/