On the two misguided occasions when we chose to fly across the country with our less-than-five-year-old child (more on the 5 year part in a second), we bought an extra ticket and used a car seat. The 'cabin attendants' where very impressed. I thought it was common sense. I've come to find out that people think carrying their children on their lap and risking their lives is "saving money". I actually heard a woman tonight talk about going on "one last free vacation" with her child on her lap before the child was "too old". I had to bite my tongue.
I'm a licensed pilot (general aviation) and commercial jets scare the $%#%$ out of me. You can not afford not to buy an extra seat for your child. If the plane hits "rough air" (what us pilots like to call "light chop") you will not be able to keep your arms around that 35 pound mass. I can show you the law of physics which make this so. No amount of caring or loving will change these equations. Buy the extra seat. Use a car seat. Or better yet, stay home until your youngest child has reached the age of five. That's my plan.
I was in a commercial plane the other day and had an unusual experience during the takeoff. The pilot pulled the nose up, realized he did not have enough air speed, put the nose back down, waited about 10 seconds and then "rotated" again. I thought I was going to wet my pants. No one else around me seemed the least be worried. Sometimes it's better to be ignorant. This episode would not have been that noteworthy expect that the plane was 3 hours late taking off because the crew had not had the required time off between flights. So this crew might have been, shall we say, fatigued. Guess what the #1 cause of airplane accidents is - fatigue combined with schedule pressure.