FWIW, we also couldn't prebook the Asiana biz class portion in advance. We got seats on the plane, but couldn't upgrade to business (even directly with Asiana) until we were actually in Seoul. They were as pleasant as can be at the business class checkin desk, and upgraded us without question. (of course, it's also possible we would have arrived and found no seats available).

On a Malaysian Airlines 747 their business class was split in to upper cabin and lower cabin. I was upstairs and it was great as there wasn't that many of us there. Check seatguru to see where is best, e.g. away from gallies. I would not worry about where to sit according to travelling with your children. They sound like good fliers and they won't be disturbing anyone else.

My last business class flight involved a 3 year old sitting opposite me with her parents. She was able to switch between each parent as one had a spare seat next to them but was entertained for the 4 hour flight without a sound. She had her portable dvd player and books and was a happy and quiet little thing.

Have fun sounds great.