Criterion-guided Evidence Chain Composition and Symbolic Reasoning for Parent-Child Interaction Video Assessment

Parent-child interaction (PCI) video assessment requires determining whether caregiver and child behaviors jointly satisfy standardized criteria governing participant roles, target identity, and temporal relations. Relevant cues are distributed across time and modalities, so retrieving relevant segments alone is insufficient to establish whether these constraints are jointly satisfied. Moreover, existing video understanding models may miss relevant behavioral cues in long videos, risking the misinterpretation of missing observations as evidence that a behavior did not occur. To address these challenges, we introduce EviChain, a framework for criterion-guided evidence chain composition and symbolic reasoning over PCI videos. EviChain organizes multimodal observations into a reusable pool of interaction chains, supporting multiple assessment criteria without repeating the initial perception stage. Each criterion is compiled into a Question Logic Program (QLP) that specifies the required evidence roles and logical constraints, guiding retrieval and incremental composition of complementary chains to fill gaps in the evidence required for execution. Sufficiency-aware three-valued symbolic execution distinguishes support, refutation, and insufficient evidence, mitigating the risk of missing observations being treated as negative evidence. Only unresolved cases are passed to an evidence-conditioned video-language verifier. We also introduce PCI-Bench, a benchmark for standardized PCI assessment comprising 243 real-world videos averaging 153 seconds in duration and 17,739 criterion-level assessments. On this benchmark, EviChain achieves 69.90% accuracy and 63.75% Macro-F1, exceeding the strongest evaluated baseline for each metric by 2.40 and 4.52 percentage points, respectively.