You may also find more attention to rules prohibiting movement between cabins on flights destined for the US. While the DHS rule exists for security purposes, it also serves to reduce movement due to Covid. All it takes is an FA not permitting it and one parent will be in J and the other in Y for the entire flight.

Thus, while you may very well be able to swap once, likely at a time convenient for the cabin crew, e.g. not during a meal service, you must plan on not being able to swap at all.