This study investigated familial attachment-based processes in middle childhood, using 788 families (50.6% boys; 84.4% White), assessed six times from 4.5 years old to Grade 6. An adapted Random Intercept Cross-Lagged Panel Model revealed between-family associations among couple emotional intimacy, relationships with both parents, and child social skills (β = .18-.66). Within-family increases in child assertion and self-control prospectively predicted relationships with parents (βs = .13), and parent-child relationships predicted various child social skills (βs = .13-.17). Couple emotional intimacy predicted child cooperation, assertion, and responsibility (βs = .12-.24) and father-child relationships in Grade 6 (βs = .20-22) at the within-family level. Findings underscore a systemic consideration of attachment-based processes in the family.