Given the fact that in this country automobiles kill over 100 people per day in crashes, and probably that many again due to air pollution, environmental degradation etc, I would ask if it is ethical to drive a car, with our without children. The question is ethical in nature, and I believe that placing society at risk as a result of your transportation choices is highly unethical. I am not sure of how may people are killed by cyclists carrying toddlers in trailers, but I am pretty certain that the number is quite low.
FWIW, I transport my grand-daughter in a trailer. Like the ethicist, my children have outgrown the trailer and ride their own bicycles now.