I seem to get several calls each year about "acute mountain sickness," which may occur when traveling to altitudes above 5,000 feet (1,500 meters) but is typically associated when traveling to altitudes of 8,000-14,000 feet (2,440-4,270 meters). To give you a frame of reference, Denver, CO, is 5,280 feet above sea level, while Vail, CO, is 8,200 feet.
Fortunately, most people will not have serious problems when traveling to higher altitudes. The human body acclimatizes to the altitude, allowing you to function with less oxygen without having distressing or debilitating symptoms. Despite that, the body is not functioning as well as it does at sea level, since the air is less dense at higher altitudes and consequently there's less oxygen available for breathing.
The first thing you may notice is a slight increase in respiratory rate, which will help increase oxygen delivery to the lungs, but at the same time results in the loss of extra CO2. Some people may also notice an increase in heart rate. I think most children without underlying medical problems (chronic pulmonary or cardiac problems) seem to acclimate better than adults.
But in some cases you may notice that your child has non-specific symptoms, such as irritability (though this may simply be due to traveling or the fact the child is simply having a bad day), decreased appetite, headaches, disrupted sleep (which always seems to happen when traveling with children) and occasional vomiting.
All of these symptoms usually resolve after several days and may be minimized by planning a gradual ascent to higher altitudes. So, driving may be better than flying, though can remember several days driving to Colorado with cranky children and we were not even out of Texas! I also think one of the boys vomited due to the driving and not the altitude.
For some children and teens who have experienced repetitive episodes of altitude sickness, I've used a prescription medication called Diamox to minimize symptoms. I would not recommend this for young children. You should speak with your doctor about the use of this medication, as it aids in acclimatization by increasing the excretion of bicarbonate in the kidney, which will stimulate the respiratory rate and improve oxygenation. Some families who are frequently sick when skiing or hiking take along portable oxygen to help alleviate symptoms for the first several days they're at higher altitudes.
For most of us, just maintaining hydration and taking the first few days of exercise a little slower than usual is enough for our bodies to acclimate and enjoy the trip.
Hubbard is a nationally known pediatrician and co-host of "The Kid's Doctor" radio show. Submit questions at http://www.kidsdr.com.