Parents of children facing cancer are subject to psychological distress. In this study, we explored whether the time that had passed since a child’s cancer diagnosis was associated with parents’ distress levels and whether parental sense of competence (PSOC) moderated this association. Forty-four parents of children with cancer who were hospitalized during 2022 participated. Parents completed questionnaires and provided hair samples for the examination of hair cortisol concentration (HCC). Correlations indicated no significant association between time since diagnosis and distress indices. Yet, linear regression analysis revealed that PSOC moderated the association between time since diagnosis and parental HCC (β = −0.36, p < 0.05). For parents with low PSOC, time since diagnosis did not predict parental HCC. For parents with high PSOC, a longer time since diagnosis was associated with lower levels of parental HCC. Our results provide support for the clinical significance of PSOC as a target for intervention.