Ok, just spent some time in the garage trying to recreate it. Locking in the trunk I think was a fluke. I remember her purse handle was caught in the liftgate, so I think it closed enough to latch, but not enough for the computer to say it was closed all the way. Therefore it didn't reopen.

However, after talking to my wife about how she locked them in the other time, here's how you can lock your keys in the car.

Lock the vehicle by pressing the roundel (like you were leaving the car to go shopping). Now open the passenger front with your hand (don't press any buttons). This unlocks the passenger front and driver front. Now, hit the roundel again with the passenger door open. This locks all the doors again. Throw your keys on the seat and close the passenger door. Congrats, you've now locked your keys in your car.

My wife was loading stuff into the front passenger side of her X5 and set her jacket down on the seat. Something she loaded onto the seat in X5 landed on her jacket just enough for the roundel button to get pushed locking the doors. She closed the passenger side door and walked over to the driver side which was now locked as was the passenger side she'd just closed.

So, while it's unlikely to happen, my wife did manage to do it, and so I'm sure children holding our keys will manage to do as well.