SFO-NRT in August should be a very full flight. They might easily need 3 seats. I would get to the gate as soon as you arrive in SFO (if the gate is open for the flight) and make yourself known to the gate agents and tell them you are a party of 3 in WBC and are willing to volunteer your seats, if needed. I have done this in the past. You cannot do it at the lounge -- you have to do it at the gate. I have taken VDB compensation and rebooked on next day flights more times than I care to remember.

VDB, though, is last-minute and there are no guarantees. I would call NW and explain that you are traveling with an infant and see if they can help you out. I am not sure what the specific rules are, but if your child is having difficulty with the pressure changes, they might be able to accommodate an unscheduled stop-over. Of course, this is last-minute, too, and is contingent upon available space on later flights, etc.