We tried to rent a 4 person surrey bike from the Boardwalk area with our kids last year when they were 10 and 8 and the CM wouldn't let us do it because he said DH and I wouldn't be able to pedal it alone. You need a 3rd person who is tall enough to pedal (and our DS who was 10 at the time was just barely below the height required). Fortunately my sister was arriving the next day, so that gave us 3 people to pedal (which was still not easy) with the two kids in the back.
So sorry, but I don't think you can rent a surrey bike this trip with 3 kids that young. For the future, we enjoyed the ride around POR.