I’m sure that this has been asked here but I can’t find the answer.
The general rule for children traveling in the company of one parent is that the second parent needs to provide a notarized letter that he or she is not opposed to the other parent to be traveling with the child.
I have provided this letter multiple times when my son traveled with my wife to Ukraine and Israel.
You can see relevant policy from your airline but here is the American Airline policy page and Sample Letter as required by the State Department
Credit:stackoverflow.com‘