In December it is extremely unlikely you will get that third seat for free, as flights that week are usually very crowded. I think it will be an absolutely miserable trip for your wife with 2 children and 1 adult crammed into 2 seats all the from Oslo to Seattle; can you change your daughter's ticket to a child fare seat? That would make all of them much more comfortable, and increase their baggage allowance as well.

If you must have a lap child, I would not risk penning the 3 of them in beside another passenger, especially as the passengers in front of them might recline during the long flights. I'd guess transatlantic the plane is a 2 aisle, with a section of seats in the center? I'd opt for an center aisle and the seat next to it to avoid having to climb over someone every time the kids need the bathroom or to stretch their legs.