Here is a picture of my daughter Lily in my 182 and she has flown a few times. She was two here but is a cerebral palsy sufferer and is very much like a 3-6 month old baby (not sure if this counts) and she liked it but the kids headset was still a bit big.
She predictably sleeps most of the way (after skillfully shuffling around to remove her headset.
Then again so does my wife and 7 year old. There is usually just me left to talk to myself and this happens even on a 1 hour flight. I must be really boring.