A 6-year-old boy is often teased at school because he has stooled in his underwear almost daily for the last 3 months. He was toilet trained at 2 years of age without difficulty, but over the last 2 years he has developed ongoing constipation. His family is frustrated because they cannot believe him when he says, "I didn't know I had to go." He is otherwise normal; school is going well, and his home life is stable. His only significant finding on examination is stool in the rectal vault. The plain radiograph of his abdomen is shown. Initial management of this problem should include which of the following?