What do I do now?

We went to Target today to get Sleeping Beauty and after I parked, I opened Makenna's door with my clicker. I usually get Jordyn out first and then go unbuckle Makenna. Before I even opened Jordyn's door, Makenna was out of the van. I know she was buckled before we left so I was a little baffled! I asked her how she got out and she casually replied, "I pushed the button!" Um.....what??? You are only three. You are not supposed to be able to get out of your car seat!! (Or is this normal?) Do kids normally learn how to unbuckle their car seats at this age? I thought that it might have been a fluke and that she wouldn't be able to do it again but she did it again when we got home. How am I going to keep this girl in her car seat while we are driving? I don't care if she gets herself out when we get somewhere. I just worry that she will unbuckle herself while I'm driving and I won't know. She is really good about seat belts. She yells at us if we haven't gotten ours on but what do I do to make sure she stays in her chair???