Originally Posted by ashill

There is absolutely zero chance that AA (or any other airline) will sit a four year old not next to one parent, and there is I think a very small chance that the eight year old will be seated not with a parent. AA, or any other airline, will move a passenger with an assigned seat at the gate if they have to to seat the four year old with a parent. (I've got a few years until I fly with an eight year old; I really don't have a good sense for when a kid is old enough to fly not sitting next to a parent either in reality or in the airline's view.) However, the OP's family may wind up as two sets of two instead of one group of four, or, at worst, 2, 1, and 1. Two and two may be preferable anyway, depending on the aircraft; obviously with the exception of widebodies with four across in the middle, four seats will at least have an aisle separating them anyway.

Therefore, I personally would not pay a seat assignment fee just to get a four-year-old seated with me; it is entirely on the airline to find a way to make that work and not charge you for the privilege, and I've never had any trouble taking that attitude with respect to a young child. (Though if you can get a seat assignment without paying in advance, that's easier for everyone, including the airline and your fellow passengers. Airlines like BA and AC that normally charge for all advance seat assignments waive the fee [by policy, not agent kindness] for young children, but I don't think AA does because they try to trick you into paying.)

If you want to get all four of you seated together, that is not something that is incumbent on the airline and is probably something you would have to pay for in practice. That's up to you as the traveler/parent.