The best advice I was given when my children were young, was to wait
until they were old enough to understand and follow directions.

Okay... we all know how difficult it is to get kids to do both at the
same time, but one or the other will suffice.

Both my kids had their first rides when they were between 2 and 3 years
of age. My son got his ride in a booster seat of a Champ. He wore a
DC10-13Y headset (and actually kept it on!) plugged into the portable
intercom.

My daughter got her ride in the backseat of a Beech Sundowner while
transporting my wife to be with her mother following my father in law's
death. The kids then rode home in the back seat while I flew. Each was
given a grease pencil and happily drew on the rear windows, which
occupied them during the hour and a half flight home.

After that trip, each time we drove past the airport, my daughter would
ask, "Daddy, are we going flying now?"

Now, when we go on trips, all they want to do is play their GameBoy's.
The one item that got their faces pressed up against the windows was
the flight along the Chicago lakeshore to and from Oshkosh last summer.