Show the decision framework: "If I am customising less than 30% of a quality parent theme, a child theme saves time and gets updates. Beyond that, I build from scratch because the child becomes harder to maintain than a custom theme."
Child themes inherit functionality from a parent theme and allow customisation without modifying the parent — safe for parent theme updates. Use child themes when: customising a commercial theme, making minor modifications to an existing theme, or learning theme development. Build from scratch when: requirements are unique, no suitable parent exists, or the child overrides so much that the parent is mostly dead weight. Pitfalls: parent theme updates can break child theme overrides if template structure changes, performance overhead of loading two themes, and debugging is harder when behaviour is split across parent and child. Strong candidates mention: style.css Template header for declaring the parent, wp_enqueue_style for properly loading parent styles (not @import), and that functions.php files are additive (child loads before parent).
Tests practical theme development judgement. Candidates who always use child themes may not be able to build from scratch. Those who never use them may be reinventing work unnecessarily. Look for the ability to assess which approach fits the project.