I would have had heart failure in your situation!!! I am so glad that they were found safe and sound. Now that I see that you have a latch handle (Can't think of the proper name of that knob) I can see how easily they could learn to open the door. My screen door has that type of handle and if I ever leave open the storm door I always lock the screen because it is easily opened even by accident.
Now listen you NAW-TEE-GIRLS!!!!!!! You stay inside that appartment, you hear??!!??