A lot of you seem certain that I would have had no problem getting my seat changed, or absent that getting two people to give up their aisle or window seats to change for middle seats so I could sit with my kids (aged 12 and 11). From the posts on flyertalk, that are a lot of people (myself included) who would not change an aisle or a window seat for a middle seat for any reason. I don't think I could even bring myself to ask.
I do agree with those of you who say it is always better to fly in the day before when going on a cruise.