I fly with my kids all the time. I always use the car seat strapped in with the seat belt. I've looked at those CARES harnesses and although would be great on an airliner with lap belt only are useless in light aircraft with the 3 point lap/sash seat belt set up. I have searched high and low for a better alternative but have found nothing.
I waited until my youngest was about 14 months before I introduced her to flying and the biggest issue I had was she wouldn't keep her headset on, we did alot of practice at home and tried various methods but in the end we went for the industrial/swimmers type ear plugs designed for noise suppression (search around online for kids sized ones) with the headset over the top when she would allow it, which she doenst seem to mind oddly enough. Ive had some suggestions of also putting a beanie on her to stop her pulling the ear plugs out but haven't needed too yet. I also fly slower when she doesn't have the headset on and drop down to around 20/20 in a C182.
My eldest was 3 when we took her for her first flight and she was a bit scared the first few times, but now we always go away on weekends in the plane and both kids love it as well as my wife (who by the way was scared of flying and vouched never to go near a light aircraft). None of my family like turbulence however so there is still a bit of work to do there.
My suggestion is make the first flight a short one to gauge how she copes and take your wife/partner/responsible adult whose sole purpose is to look after the infant.
Good luck with it.
If you need any other advice just pm me.