Arrive at the airport early to leave enough time to make last-minute adjustments, if needed: “Airlines will do what they can at the airport to help families who self-identify to their agents as needing to sit together.  Even if the airline is unable to seat the whole family together, they may be able to assure you that each child is seated next to an adult family member.”

After traveling, report any problems to the DOT: “If you should have a problem during your trip, you may file a complaint with the airline or DOT. The feedback you provide to the airline may influence the way an airline interacts with families traveling together.  … The Department also sends every family seating complaint it receives about an airline to that airline. Airlines must respond to written consumer complaints. Your comments or complaint will be reviewed by the Department and the airline.”

What the DOT has not done is implement any rules requiring airlines to seat families together. And it’s evidently up to you to pick the correct airline and cabin class that won’t charge you for seat selection, and even then you might need to work with the airline to make sure you’re seated together. Here are the airline family seating policies, according to the DOT, for nine airlines: