You must get a passport for your child or children (under 16 years) if you plan to fly to another country. In the case of air travel, the United States will not allow any person (including children) to enter or re-enter the United States without a valid passport. If you are traveling by land and sea outside of the United States, all persons must present passport book or passport card or other authorized travel document to enter or re-enter the United States. The passport card is a special card valid only for land and sea travel between the U.S. and Canada, Mexico, the Caribbean region, and Bermuda. To view more on this and the United States authorized travel documents for land and sea travel, visit: http://travel.state.gov/travel/cbpmc/cbpmc_2223.html. To view the United States’ requirements for obtaining a passport for minors, visit: http://travel.state.gov/passport/get/minors/minors_834.html

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Where can I use the passport card instead of a U.S. passport?
If I need a replacement passport for my child, do both parents need to sign the application?