What would the police do?

recently someone has been taking our mail and we were scared because my passport was coming so we got a mailbox that needs our lock and as soon as we got it the passport came, this morning i was in a room closest to the mailbox and through the window i saw this little girl opening it, i know it's her because my mom ( im 13 -__- ) said that she saw a little girl hanging around it, if i called the police and reported her what would they do?