I don't know the answer to your question; however, I can tell you that several of my friends fly with their young children. I'm not sure what precautions they take, but one thing to keep in mind is that most airliners are pressurized to a cabin altitude equivalent to about 8,000 ft. MSL at cruise altitude. As long as you aren't traveling through the mountains in a small plane, it's unlikely that you'll spend any time at a cabin altitude higher than you would on a commercial flight. So, as far as pressure and oxygen goes, the experience won't be much different than a commercial flight. (Be sure, however, to confirm this with the pilot.)

I can think of two things to watch out for, however:
1) If you do determine that it is safe for your child to fly, be sure to provide adequate hearing protection if you are flying in a plane with a loud cabin.

2) The most uncomfortable time, with regard to the child's ears, will be during the descent. The more gradual the descent, the better (if the pilot can accommodate). Also, be sure to avoid flying if the child appears to be ill which may cause the sinuses to be congested.