My son did the same thing at that age. He only did it once, though. I pulled over immediately, spanked him (I felt it was justified considering how dangerous it was), and told him quite forcefully that what he just did was very dangerous and that he was never to do it again. When he saw how upset I was, he decided not to try it again.
Another thing you can do, is allow her to unlatch it herself when you are ready to get out of the car. Just make sure that she knows she has to get the signal from you first or she will have to face some negative consequence (decide what that is and be consistent). This might fulfill her need to perform her new skill in a way that is safe and mom-approved.