The argument would be made that there is no way the child could be kidnapped on a plane (unless the kidnapper had a parachute).
Nevertheless, this is neglect and irreponsibility. I have a 4 year-old and 2-year old. I don't like leaving them unaccompanied in their own bedrooms, let alone an airplane!
If it were me, I would have said something.
If it were my wife, she would have caused internal injuries.
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"I will not be pushed, filed, stamped, indexed, briefed, debriefed or numbered. My life is my own."