This sounds like a bad plan. No carrier will ticket a 3 YOA into a separate cabin, especially on departure from the US. What exactly happens in an emergency when the parents are in F/J and the 3 YOA is in Y? Too young to be an unaccompanied minor and too young to help himself with parents located too far away.
Rather than making plans and then calling, why not contact the carrier now and explain the child's needs without getting into who sits where and does not. The carrier will explain the limits of its capabilities and you can then determine whether those will work.
Issues to confront are: sepatate cabins and not seated in own seat for taxi, takeoff, turbulence, and landing. The latter are potentially serious physical risks to the child and to other passengers.
Last edited by Often1; Oct 8, 2017 at 10:21 am