Well, if you are trying to 'minimize' your costs on this flight, you can by not selecting seats next to you or your spouse, your children will possibly be placed in a seat upon check-in. Then you are at the mercy of some 'kind' passenger willing to change, low possibility if they have booked AND paid for the seat they are in AND if your childs seat is in an undesirable area for that passenger.

Airlines which pledge to not separate children from parents, do this in the US are doing it as an accommodation and not regulation.

As for Global Entry, the airline has to participate.

Safe travels