All good posts above. Our boys have been flying since before they were born. They are now 6,4 and 2.
One note of caution flying with small people, especially infants, is to watch your rate of climb and descent. While it is an easy thing for adults to clear their ears, equalising pressure, not so for the small people. Food helps on multiple levels!
As mentioned previously, kids can change their perspective to flying quickly. Our 4 year old, having spent three years flying regularly looked down one day and decided that he wasn't keen anymore. We didn't push it and let him sit out the flights for a while. Then when he decided he wanted to go back up, we made sure he went out on blue sky days with no turbulence, until now he's happy again riding in the plane.
A second adult is a great thing in the aircraft when the kids are young; to help put the headset back on, hold a hand, clean up that liquid, where ever it comes from, dole out food and generally run cover for the one flying.
Car seats are great, and hold them well.
None of our kids wanted to wear headsets when they were small, until flights in helicopters made them change their minds.
1+ on the value of Pilot isolate on the comms box!
Now the big thing is who gets to sit right in the back, far from their parents 'helpful' reach.
