To The OP - If its more about baggage, then is the excess bag charge more than an extra ticket for your destination and dates ?
Usually, airlines are reasonably flexible even with ensuring the adjacent seat is kept empty IF the airline is not full in my experience..(also because the baby can be noisy and many pax would prefer other seats)..

If its only a domestic leg as you mention (~ 2 hrs), I would not worry much but YMMV..