...you need to cancel a flight
As far ahead of the departure time as you can, inform the airline that you aren't going to make it. After paying a penalty of $50 to $100, you can apply the remainder of the fare to a flight in the next year. Many carriers are sympathetic to illness-related cancellations, so mail your carrier a doctor's note-you'll likely be reimbursed.
...your child has a stuffy nose
In the case of a bad cold, or one accompanied by a fever, reschedule the flight. For common sniffles, clear the child's head before boarding: Kids 14 and up can use decongestants; nasal saline sprays help relieve pressure for younger kids. Encourage swallowing during takeoff and landing; sipping water and even making funny faces does the trick.
...you're concerned about your trip's effect on the environment
Fly direct routes (takeoff and landing require the most fuel); ride in economy class (less legroom packs in more people); and buy carbon offsets from reputable programs such as nativeenergy.com.