The MP5s I have seen in use are fairly accurate, it just depends on the distance used. 200m is a stretch but 100m or less (which a airport is) I would be very confidant with a MP5 even on full auto. The groupings I have seen boarding party personelle do with a MP5 is absolutely fantastic at 25m even on full auto.
9mm doesn't have the same range or ricochet potential as bigger round (a plus in this situation) and it is more controlled through a short barrel carbine than it is through a pistol. Next to no recoil means it is easier to control, and your argument that they are inexperienced is relatively irrelevant as no matter what you gave them if they don't know what they are doing they are a risk to all around them.
If you can think of a better option for arming them let us know, otherwise the MP5 is basically the best firearm they could have picked for that type of area.