Well, you guys just won't give it up will you?! Here's the point....saying something or doing something, to another passenger when you aren't able to be seated with your child (here's the diaper bag, have at it, or she gets airsick so I'll give you a bag to use when it happens) is way uncool. Generally speaking, and I believe this was kaytieeldrrs point, the airline does not want to split you up. IF it is unavoidable, they will ask someone to change their seat so as to enable you to sit with your young child. BUT...it is not the passenger's fault if the airline places your child next to them....it is the airlines fault. So why would you want to try to punish the poor passenger who has the 'honor' of sitting next to your child??? That passenger has done nothing wrong...they had no idea there was an issue finding seats together for a parent and child..they merely got on board the plane, sat down, and got settled. Now...some woman comes up, plops her toddler down next to them, and says..'here's her diaper bag. she has a tummy bag so you can find clean diapers in there quickly.' Can you just imagine??? That is what I believe the point was.
The point is this...yes, you get to choose seats ahead of time. The informed traveler realizes that seat assignments can, and do, change..and sometimes you don't know about it before you get to the airport. Fair? Not really, but the airlines do put a disclaimer in their contracts of carriage. We just never read the fine print.
So...this thread is done. There is, and never was, any reason for the venom that occurred. You are free to disagree but it has to be done in a polite manner. The OP has gotten the info she needed. Stick a fork in this one, it's done.