I would head over to the Incheon Hyatt for their spa, have a leisurely soak and possibly massage there and take the bus to Seoul at around 9am when the worst traffic jams are over. If you want to see palaces and traditional markets head north of the Han river, if more interested in a/c:ed shopping malls and modern architecture then south. Just leave enough buffer on your way back, traffic is unpredictable.

Seoul is far from an ideal sightseeing destination, especially with a toddler, so I would seriously consider spending part of the day at the Hyatt ( spa & very nice lunch buffet ) and then heading into the comfy OZ lounge for resting before your onward flight.