I asked the AF FAs on my flight back to the states and they said that this behavior was not allowed and shouldn't have happened.

Personally, I am less concerned with the terrorism threat and more concerned with having 2 teenagers in the cockpit during the sterile part of the flight. Imagine if something happened and they start freaking out. As we saw with the miracle on the hudson crew don't always have enough time to execute the manual by themselves, and they forget stuff (ditch switch). Imagine what could happen with 2 observers adding to the stress of the flight.