It is not mandatory that you have a signed letter from the other parent, as long as the border officials are satisfied that you have proper approval from that parent, or a court, to travel with the child.
Your child will need a Child Travel Consent any time they're traveling without all their parents or legal guardians present. If your child is traveling alone, with only one parent, or with a non-legal guardian, they will require a Child Travel Consent signed by both parents or legal guardians.
A consent letter is recommended for all cross-border travel, even for a day trip, if the child will be travelling alone, with only one parent, with friends or relatives or with a youth group.
Is a minor travel consent form required for domestic travel? No, a minor travel consent form is not required for domestic travel within the U.S. However, it is recommended that parents and/or legal guardians carry a notarized letter of consent when traveling with a child.
The United States Does Not Have Exit Controls: Two-parent consent is not always needed for a minor to leave the United States. Without a valid court order prohibiting the child's travel outside of the United States, there may be no way to prevent an abduction.
Unless your specific court order prevents out-of-state travel or other restrictions, generally each parent may choose to travel.
It's important that you give your co-parent notice and preferably get their express permission in writing if you wish to travel outside the state with your child. If the other parent refuses to cooperate, you can ask for the court's permission in advance.
A letter of consent, preferably in English and notarized, from the other parent or signed by both custodial parents. The letter should say: "I acknowledge that my child is traveling outside the country with the name of the adult with my permission."