["Family Relations, EarlyView. ", "\nABSTRACT\n\nBackground\nGrandparent–parent co‐parenting is common in China, but its link to preschoolers' problematic media use is unclear. This study examined how co‐parenting patterns are associated with children's media use, focusing on the mediating role of parental digital parenting self‐efficacy.\n\n\nMethods\nA cross‐sectional survey of 1910 parents was conducted. Latent Profile Analysis identified co‐parenting patterns, and mediation models tested their relationships.\n\n\nResults\nFour patterns emerged: Supportive‐Harmonious, Involved‐Competitive, Conventional‐Disengaged, and Conflicted‐Dysfunctional. Children in the Conflicted‐Dysfunctional group had the most severe media use problems. Parental digital parenting self‐efficacy was a key mediator, fully explaining the association of the Conflicted‐Dysfunctional pattern with problematic media use, and partially mediating its associations with other patterns.\n\n\nConclusions\nThe quality of grandparent–parent co‐parenting is an important correlate of preschoolers' problematic media use. Conflict and disengagement in the family are associated with an increased risk of excessive or problematic media use, largely via their association with reduced parents' confidence in managing their children's digital habits. Interventions should go beyond the parent–child relationship to encourage supportive co‐parenting and bolster parental digital parenting self‐efficacy in multigenerational households.\n\n"]