Crossing state lines, even on vacation, can lead to unintended consequences. The lesson: Know the rules and your rights before you go.

The first time someone whispered it to me, I was boarding a Nashville-bound flight with Fred, a toddler, and Cheryl, an infant on the hip. That trip is known in the family saga as the “Coupon Flight,” because 15 of my unexpired drink coupons bought rounds for the folks near Cheryl as she wailed her way across the Mississippi and into Music City. I’ve heard the axiom repeated several times since: There are two ways to travel: First Class OR With Children.

Despite the myriad frustration of any travel in these post-9/11 days, the lucky few who luxuriate in wide lounge seats cannot expect to earn a place on Oprah Winfrey’s sofa by whining about the long wait for luggage. Brag about your sea kayaking off Baja, your climb of Kilimanjaro. It is travel with children that deserves our respect. Add airline regulations, government mandates, and custody orders to the packing list and only the bravest will venture across the seas or around the world.

With the blended family often come mandated parental visits, overseas grandparents, important allocated holidays. Gone are the days when a quick casual note and a photo ID from the paintball park are sufficient to fly a child down to Cancun for spring vacation or up to Quebec for a quick summer trip with Aunt Mary.

It starts with passports. Restrictions tighten and ease, but last summer’s rules required passports for flights to and from all nearby countries including Mexico, Canada, and the Caribbean. The pace of distribution of passports has picked up, but some changes in the guidelines may surprise unsuspecting parents. Most travel agents suggest allowing 4 to 6 weeks for new passports as well as renewals. The government will expedite for a fee (a hefty one), and private companies, such as RushMyPassport.com, may work in days, but it’ll cost even more.

Just getting a passport for the children can be a trial. As of February 2008, both parents of children under age 16 must appear together, offer proof of relationship to the child, present proof of their own citizenship, and actually sign in front of an authorized government agent. More than 9,000 designated post offices and other government agencies are listed at the website www.travel.state.gov/passport. More user-friendly and better written than the IRS site, it provides organized information about passport application processes and requirements as well as downloadable forms.

Suppose parents cannot, or will not, appear together at the signing. Then one parent must appear with the second parent’s original notarized statement of consent. In rare circumstances, this requirement may be satisfied by additional documentation, such as court orders giving unlimited sole custody, judicial declarations of incompetence, adoption orders of sole parenthood, or oath certificates.

The State Department requires that any parent who takes children ages 18 or younger out of the country without the other parent must have specific, detailed authority to do so. Add airline regulations on top of that: A random check of U.S. carriers indicates most also insist on notarized permission, along with proof that the parent has the right to travel with the child. If the receiving country refuses entry, the airline has to fly the child back home. Some countries also require that the permission authorization be translated into the receiving country’s language. Most countries additionally call for the passport to be valid for 6 months past the anticipated date of departure from the country.

At present, those traveling by land and sea may show proof of citizenship by means other than passports-for instance, the fairly new government-issued passport cards or a certified, original, nonlaminated birth certificate. Social security cards do not satisfy the requirement. Bringing along the birth certificates-even for days’ old infants-is good practice, even with valid passports, to ease the process of replacing lost papers.

It is critical to scan individual airline websites for distinct requirements. Most mandate that parents (or those traveling with the child) have proof of relationship to each child. This proof is especially critical when the traveling parent and child have different last names. A parent’s name change must be documented from a primary source such as a marriage certificate. In an era of multiple remarriages, that can be time-consuming at the gate. Think of the drama that unfolds as the bonding family checks in: loving couple with great intentions, his two sons from his first marriage, her daughter from another marriage, and their shared child from the current union. But the youngest left his photo ID at home. The day when the airline personnel can just smile and wave a passenger through the gate is long past. More than one family vacation has been ruined as one parent is left behind with a poorly documented child while the rest of the family boards the plane.

When divorced or separated parents don’t agree on foreign travel, or if a court order doesn’t specify, a family law judge may have to decide the question. Occasionally, parents want to travel with children to countries that have yet to ratify what is commonly known as the Hague Convention. (This is an international treaty whose signatories have reached agreement about enforcing the custody orders of other subscribing nations.) Some courts will require the parent with ties to a non-Hague country to post a bond, sizable enough to pay the other parent the costs of seeking the child in the event the traveling parent fails to return him or her. Movies have been made and books written about children taken to countries where the objecting parent has no legal standing to force the child’s return.

Parents who have sole custody, joint legal custody, or an order prohibiting the child from traveling without permission may ask to have the child’s name entered into the State Department’s Children’s Passport Issuance Alert Program (contact at 202-736-9156, Office of Children’s Issues’ Prevention Unit). Once alerted, the State Department will not issue a new or renewal passport for a child, but this does not invalidate an existing passport. Where this is a concern, it may be worthwhile to seek an amendment of current court orders.

The only things that we like to think can spoil a trip are rain, faulty air-conditioning, or lost luggage. But on the rare chance that a medical emergency threatens the health of a child, the traveling parent is well advised to secure a notarized statement from the other parent granting authority to request medical or surgical care on behalf of the child, plus carry copies of prescriptions for eyeglasses and for routinely taken medicine.

On the other hand, that neighborhood wading pool is looking more and more like a first-class place to spend the week.

A former family court associate judge in Houston, Texas, Patricia Lasher has specialized in family law for more than 20 years. She has written for numerous consumer magazines and has published a collection of profiles, Texas Women: Interviews and Images. With her own remarriage, she now divides her time between Baltimore, Maryland, and Houston.