I've run into this as well---I normally don't split since we have four, and one upgrade doesn't help if it leaves the other parent with two kids, but did split b/c booked award tix out of separate accts to clear out spouse's slow-growth WP miles. I wasn't that surprised, which may say more about NW's service than the change in the first place...
The one exception is a minor companion, who will be seated with you.