People asking me to trade seats is one of the things that drives me nuts the most. At best my trips are planned 14 days in advance but usually less. I still am able to in most cases, procure a seat that I like. Most family trips are scheduled multiple months in advance to coordinate parent's vacation time, kids out of school, grandma's schedule, etc. If the family cannot coordinate adequate seating then I am obviously a better planner. I should not be penalized for this skill.
That said, here is how I handle each and every seat change request. First, if you are asking me to trade, offer something better than what I currently have. Don't ask me to give up an aisle for a center or window. Don't ask me to move towards the back of the plane. If these are the options I simply reply with "no, I don't want to give up this seat for a seat of lesser comfort".
Second, survey the landscape. My exception to the rule above is based upon accurately making this assessment. If I am going to end up keeping my ailse seat yet sitting next to a mother with a lap child, all rules are out the window. I love my kids but no one elses. I don't in anyway want to spend hours next to a lap child that is going to be uncomfortable, cry, fidgit, possibly emit body fluids, etc. Also, I assess the landscape of the seat I am being asked to take. Again, I don't want to move up two rows and end up next to a POS.
My flight back from SIN yesterday, there were two ladies that tried to manipulate an entire 5 seat center row. They preselected each aisle and as other PAX appeared in the row, one of them would trade and move towards the other end. They obviously were trying to keep as many empty seats between them as possible. They ended up sitting next to each other with no empty seats between them. This is fine except that after the first trade, they no longer had any seats that were in their direct control. They were asking other PAX on the row to trade between themselves, litterly acting as though they owned the row. I was very glad I was not in the row because I would not have made them very happy.
Overall, maybe I am cold and calloused but I do not mind nor do I feel guilty just saying "no, I preselected this seat for personal reasons".