I think the problem isn't so much 'someone would've noticed the girl'...

It's the fact that the bad guys' whole plan would've hinged on it. It's possible that on a flight everyone would be too wrapped up in their own business to notice the kid but it's also possible that just one random passenger would walk past them, notice that the little girl looks just like her own niece, etc, and then the whole plan is gone. They would've put in all that effort, time, God knows what else, on the hope that not one single person on the plane would happen to notice and remember seeing the girl. I actually saw this movie in the theater and enjoyed how tense it was. But looking back, that's a pretty big plot hole.