It depends on how you set your restrictions. Check your setting. there is an option to make it ask for password evry time a purchase is being made.
iPhone, iPad, and iPod touch: Understanding Restrictions
iTunes: Using Parental Controls
thank you very much with your help. for the password for restrictions. i never set it, so how do i know whats the default password? thanks again
by the way, i try go "machines" which the place i purchase i pad, the sales person told me the i-phone or i pad only will ask password "1" time only.....