This study examined the association of family and maternal characteristics with preschool children's dietary patterns. Trained interviewers evaluated subsample 3422 mothers and children enrolled in the population-based birth cohort Generation XXI (Porto, Portugal, 2005–2006). Maternal characteristics and behaviours (exercise, smoking habits, diet and child-feeding practices) and family characteristics were evaluated. Maternal diet was classified by a dietary score, and children's dietary patterns were identified by latent class analysis. Odds ratios (OR) and confidence intervals (95% CI) were estimated by multinomial regression models. The analysis was based on a framework with four conceptual levels: maternal socio-economic position (SEP) at 12 years, maternal socio-economic and demographic characteristics at child's delivery, family characteristics and maternal behaviours at child's 4 years. Three dietary patterns were identified in children: high in energy-dense foods (EDF); low in foods typically consumed at main meals and intermediate in snacks (Snacking); higher in healthy foods; and lower in unhealthy ones (Healthier, reference). Lower maternal SEP had an overall effect on children's diet (low vs. high SEP; EDF, OR = 1.76, 95% CI: 1.42–2.18; Snacking, OR = 1.73, 95% CI: 1.27–2.35), while maternal education was directly associated with it (≤9 vs. >12 schooling years, EDF, OR = 2.19, 95% CI: 1.70–2.81; Snacking, OR = 2.22, 95% CI: 1.82–3.55). Children whose mothers had worse dietary score were significantly more likely to follow unhealthier patterns (first vs. fourth quartile; EDF, OR = 9.94, 95% CI: 7.35–13.44, P-trend < 0.001; Snacking, OR = 4.21, 95% CI: 2.94–6.05, P-trend < 0.001). Maternal diet was the key factor associated with children's diet, above and beyond socio-economic and demographic characteristics, accounting for one-third of the determination coefficient of the fully adjusted model. At preschool age, interventions should give a particular focus on maternal diet and low SEP groups.