I first took my eldest flying when he was 6 (and a half).
I agree with the booster seat and would also make a few other points:
- Find out how sick bags open and seal before you leave the ground (wasn't a problem for him, fortunately)
- Ensure that he goes to the loo just before take off (halfway back from IOW @ 3000' - "I need a wee"...)
- Fly a few circuits before departing on a long leg to see how he reacts to the various phases of flight.
- Ensure that he is supervised on the ground while you are doing your pre-flight.
I also explained what we were doing / going to do and ensured that he understood that I would sometimes need him to be totally quiet so that I could use the radio - we had a signal for that.
He was actually very helpful, holding PLOG (and filling it in), maps, water etc.!!
Enjoy the experience.
FBW