To examine the association of family functioning with child dental behaviours and to identify family functioning domains associated with those behaviours. Cross-sectional data from the East London Oral Health Inequalities (ELOHI) study were analysed in a subsample of 733 parent-child (3-4-years-olds) dyads. Family functioning was measured with the 60-item Family Assessment Device that yielded a general functioning score and six domain scores (roles, communication, problem solving, affective involvement, affective responsiveness, and behaviour control). Child dental behaviours were sugar intake, dental attendance and toothbrushing frequency. The association of family functioning with each dental behaviour was assessed in logistic regression models adjusted for confounders (parental sociodemographic and child demographic factors). Unhealthy general functioning was associated with greater odds of reporting high child intake of sugars (OR: 1.78, 95%CI: 1.01-3.13) as well as lower odds of reporting frequent child brushing (OR: 0.76, 95%CI: 0.50-1.18) and a child visit for dental check-up in the past year (OR: 0.98; 95%CI: 0.62-1.53), after adjustment for confounders. Unhealthy functioning in roles, affective involvement and behaviour control were associated with high child sugar intake whereas unhealthy functioning in roles was inversely associated with frequent child toothbrushing. No family functioning domain was associated with child dental attendance pattern. Healthy family functioning was associated with more favourable child dental behaviours. How a family functions, particularly in terms of how they define roles and support each other emotionally, is likely to be relevant to child oral health.