The schedule is here. Use the red schedule on the right as that is down river and will be faster.
http://www.kdrhine.com/rhineschedule.htm
Best would be to drive to Bingen and take one of the boats like the 10:30 or 11:30 boats to Boppard in order to get the best of the Rhine Gorge. It takes 2 hours and 20 minutes. Then walk up to Boppard station and take the train back to Bingen Stadt then walk back to the KD dock to get back in the car.
Current schedule, there are trains returning at 1:07 pm, 1:15 pm, 2:07 pm, 3:07 pm, 3:15 pm. The departures at 15 past the hour require a connection at Bingen Hbf. to the local train to Bingen Stadt, one stop away. Takes 50 minutes or less. The train tickets for the 4 of you should be 20.40 EUR total.