The British public generally adhered to COVID-19-related restrictions, but as the pandemic drew on, it became challenging for some populations. Parents with young children were identified as a vulnerable group. We collected rich, mixed-methods survey data from 99 UK-based parents (91 mothers) of children under 12, who described their lockdown transgressions. Household mixing was the most prevalent broken rule. Template analysis found that rule breaking was driven by ‘ingroup-level’ prosocial motivations to protect the mental and social health of family and loved ones, and that parents were ‘engaged’ decision-makers who underwent careful deliberation when deciding to break rules, making trade-offs, bending rules, mitigating risks, reaching consensus, and reacting to perceived rule injustices. Cumulative link models found that the perceived reasonableness of rule violations was predicted by social norms. Rules were broken by parents not for antisocial reasons, but for ‘ingroup-level’ prosocial reasons, linked to supporting loved ones.