Yes, you can book flight seats together, but in 2026, the ease of doing so depends on your fare class and how early you plan. Most "Full-Service" airlines (like British Airways, United, and Emirates) allow you to select and pay for seats at the time of booking to guarantee your group stays together. If you choose not to pay, their systems are programmed to try and seat passengers on the same booking together during the automated check-in process. To maximize your chances for free, you should check in the moment the window opens (usually 24 hours prior). For families, many airlines now have "Family Seating" policies; for example, American and TUI guarantee that children under 12 or 15 will be seated with at least one adult from their party at no extra cost. The notable exception in 2026 is Ryanair, which uses a randomized seating algorithm for those who don't pay; if you want to sit together on Ryanair, you almost certainly have to purchase a seat assignment or one adult must pay to sit with their children.