I have 2 kids under the age of 4 and they have pretty much traveled exclusively in FC thanks to upgrades (or miles used). And no they are not well-behaved (usually) because kids that aged are rarely well-behaved and there isn't always a lot the parents can even do about it (except try). If US Air had any problem with this they'd ban children from FC, since they haven't your complaint is with them, not the parents. It's one thing if an 8 year old kid is misbehaved but a 2 year old is a 2 year old and sometimes there is nothing you can do.