That included challenging adults; in fact mostly it was directed at challenging adults, particularly those in positions of authority. I tended to ignore "children should be seen and not heard". I was blessed to be educated by teachers who considered empowerment to be an essential part of education, both believing you could do something, and then thinking in terms of HOW.
What would you do?
I would have leaned over across the aisle, and asked the man his name. Using his name, I would have then told him to knock it the hell off.
I don't think he would have stopped; upsetting someone else seems to have been part of his thing, his kick, as an exhibitionist. It's not just a sex thing; it's a power thing, a control thing.
I would then have asked for the largest possible glass of cold water, stood up, walked over, and dumped it on him. And grabbed his blanket. Some combination of that would have exposed the man, and possibly made him stand up. A photo would be a good idea, as evidence; but if he is observed by an entire airplane cabin of people, hardly essential.
Then I would have announced to the passengers and crew what this man was doing, demanded the captain come back and deal with him, and called for an air marshal to arrest him if there were any on board or otherwise to have law enforcement waiting at the nearest airport to remove him from the plane.
I did something similar when I was 14, although it was not on a plane. Because this kind of experience begins with the victim of the deviant feeling sick and small and helpless. Someone trying to make me feel that way has a very short switch flip to mad-as-hell, which usually leads to some action. Adrenaline can be a great activator.
We don't teach that enough to children, and particularly, we don't teach it enough to women and girls. Helplessness is to some degree learned. It can be UNLEARNED.
I think this young girl suing the airline is fantastic. That too is learning to be empowered.