I asked the greybeards in the EAA chapter I belong to for their advice
on when to take my first born up. The advice was to wait until they
could understand verbal instructions.

I purchased two DC 10-13.4Y's to fit their small heads. I took my first
one up in the Champ when he was two. It made for an interesting photo of
a child in a car seat in the front seat of the Champ.

My second child got to fly at about 18 months when my wife's father died
and I had to fly her to be with her mother. I flew home with both kids
in the back seat of the Musketeer. Each had a grease pencil and drew on
the side windows to amuse themselves during the 1.5 hour flight.

Following that trip, whenever we drove past the airport, the second
child would always ask, "Are we going flying?"

13 years later, the second one likes to fly with a camera in hand.
Some of the first pictures she took were of the Chicago skyline as we
flew the shoreline coming home from Oshkosh.