If you want to take a niece or nephew under the age of 18 on a trip in the United States, you must have their parent's permission to do so. If you do not have parental permission before taking a child -- even a relative -- you could be accused of kidnapping. You don't legally need to have their permission in writing, but having a notarized document with you could smooth your travel plans if anyone questions you along the way.
If you want to take a minor niece or nephew on a trip, you must have their parent's permission, even if they are a teenager and want to go. Parents have control of children living at home in most states until the age of 18, so children up to this age can't make decisions for themselves without their parent's consent. You should have permission from both parents, even if they are not married or divorced, just in case arguments arise between the parents.
Because it is easy to forge signatures, the best documents are those that are notarized. To have a document notarized, the persons signing the document must show legal identification, such as a driver's license. Before taking a minor child with you, have both parents sign a letter stating that you have permission to travel with the child and have it notarized. Many banks have notaries on staff that will notarize documents are at no cost or for a nominal fee. Include the child's name, your name, dates of the trip and the full names of both parents, Lawyer.com advises.
In most cases, children under age 18 do not need to show identification when flying domestically in the United States, according to the Transportation Security Administration. It's unlikely that the airline or TSA will ask for any identification or a letter that gives you permission to take them on a plane for your niece or nephew. While you do not need to bring a copy of their birth certificate or a letter of permission, it never hurts to be overly prepared.
If your niece or nephew needs medical care during a domestic trip, having a copy of his insurance card and a notarized letter stating that you have permission to obtain medical treatment for him can facilitate the process. The American Automobile Association recommends including the child's medical insurance information, social security number and the full names of all adults, both those traveling and the parents, in a letter. Alternatively, you can call his parents and have them give the information over the phone, but this could delay treatment if it is not a medical emergency.