[airhead]

To answer your question, "How do they come up with the seating assignments?" is indeed a very good question and I will do my best..Not all airlines operate the same but this will give you an idea.

Let's take an airplane with 100 seats. All of these seats are empty at the moment, no reservations in other words.

The first 80 people who reserve get to choose from 80 seats. 20 seats are always blocked because the Department of Transportation requires enough seating be available for those with disabilities, the word handicap is prohibited. The emergency exit row is also blocked until about 24 hours before departure.

Some airlines will let you specify age of the passenger during reservation because a child is not allowed to sit in the emergency exit rows.

The remaining 20 are left not able to make a seat assignment. The gate agent is then required to figure out who is where. The changes for safety are made at that point. It makes sense.

People often make mistakes when checking in, so do the agents at that point. Mistakes are also made during the reservation creation. Many neglect or just don't know how to claim that a passenger is under age. The computer does not know better.

So the stress comes to the gate agent to rearrange passengers (some are not happy about this) so that a child can be in a safe spot with the parent. Also, some aircraft seats do not have the extra emergency O2 mask for a lap child so that also has to be considered.

I hope this helps.