Sam and Ted have flown.  The fare was $80 EACH, ONE WAY.  I had them both, as babies in a pet carrier (soft side) for under the seat.  On the way to VA, the employees were accommodating, helpful, offering to get them water !!  Coming BACK to Ohare was  a different story.  It's all about the people.  Ted was on my lap to VA and no one cared.  On the flight to Ohare the carrier stayed under the seat (although my head was down there most of the time checking on them and baby talking them).   I paid quite a bit for them to be shoved under the seat, and 
yes sometimes the screaming, yelling, jumping 5 year olds should be put in cargo. (For they upset my dogs.)  Traveling today is rough enough, I have to sit there for two hours and listen to these kids have tantrums while their totally out of touch parents ignore it?? Sorry. I have had more flights ruined because the kids' owners couldn't control them and didn't care at all about the others around them.