I am not saying it is appropriate to show Alex Cross (haven't seen it), but if the choice of films is based upon input from each of the parents on a plane, we are going to be left with nothing but G-rated films in order to dumb it down to the lowest common denominator.
I certainly wouldn't advocate showing R-rated films, but assume that for the general audiences on airplanes, PG-13 is acceptable.
If not, how would you have United censor the films to make them acceptable?
It's a slippery slope once you start down it.