When Rhode Island real-estate agent Benjamin Thomas was offered the chance to list a $3.25 million house, his first reaction was dismay rather than elation: The seller was his mother, and the listing was the beloved waterfront home that had been in his family for about 60 years.

When selling a home, it can seem like a no-brainer to choose a real-estate agent who also happens to be the owner’s child. By hiring their child to market a pricey home, parents can boost their child’s career and provide financial help in the form...