A lot of people will say to a family, just sit separate, the the recent UA flight reminds us why that is not always a great idea. If you are sitting next to someones child and refuse to change seats and the person who the parent is sitting next to refuses to change seats, if something happens, are you going to make sure that child is safe?
Just something to think about...
As far as adult requests, I don't switch unless its equal or better. No I am not going to give you my exit seat because you are tall. No, I am not giving you this bulkhead seat so you can sit next to your brother, why doesn't your brother move in back? Oh it's because you want the better seats, no thanks.
As far as I go, traveling with family, I would never except someone else to move for us. If we got split up, we would hang out for the next flight, or spend another night to make sure we were seated next to each other.