Amy and Steve Harmon have found success by urging their 2-year-old son, Nathan, to use his child-sized potty as soon as he gets up in the morning and every two hours throughout the day.

Based on Nathan H. Azrin’s book “Toilet Training in Less Than a Day,” the woman who is called the “Potty Whisperer” trains parents as much as she teaches the toddlers. Her No. 1 rule for ...