Introduction - The COVID-19 pandemic has exacerbated social inequalities in health. School closures and the first lockdown in France took place between March and May 2020. However, no French study has specifically examined the impact of living conditions during this lockdown on the mental health of children and adolescents. The objective of this study was to assess the emotional state and psychological distress among children aged 9 to 18 during and after the lockdown, based on their environment, living conditions, and housing situation. It also aimed to measure resilience and the strategies implemented to maintain their well-being. A unique feature of this study is that it gave children and adolescents a voice, took into account the diversity of social situations, and was also extended to include young people in the care of child welfare services. Methodology—The study was conducted via an online questionnaire between June 9, 2020, and September 14, 2020, using a parent questionnaire and a child questionnaire that collected sociodemographic data, information on living conditions, and details on the impact of the lockdown. It was designed by an interdisciplinary team with a strong focus on measuring social inequalities. Mental health was assessed using psychometric self-report tools. Bivariate analyses were performed using chi-square tests to describe distress and emotional state, and Student’s t-test or ANOVA to describe resilience by age and sex, according to living conditions, parental characteristics, and activities. Results—In this descriptive study, 3,898 children and adolescents were included, among whom 81 were in the care of child welfare services. Adolescents appeared to experience greater mental health impacts compared to younger children, and girls appeared to experience greater mental health impacts than boys during and following the lockdown. Factors associated with psychological distress included housing conditions (being confined in an urban area, in an apartment or a house without a yard, lacking access to an outdoor space within the home, living in overcrowded housing, and being unable to isolate oneself), economic conditions (financial and food insecurity, reduced income due to the pandemic or lockdown, parental unemployment, lack of internet access), parental characteristics (single-parent families, educational level at or below a high school diploma, parents who are blue-collar workers or office workers, parents born abroad, and lack of social support). A lack of activities, an increase in time spent on social media and screens, a feeling of being overwhelmed by schoolwork, a close relative contracting COVID-19, and hospitalization due to COVID-19 were also linked to distress. Conversely, better living conditions, a two-parent family structure, higher parental educational attainment, social support, and engaging in activities during lockdown were associated with a higher resilience score. Conclusion—Financial support for single-parent families, the continuation of after-school activities, and regular outings are factors that can influence the mental health of children and adolescents during lockdown. Support for young people whose loved ones have been infected with or hospitalized due to COVID-19 should be promoted, as should initiatives to disseminate accessible and age-appropriate information about the situation to parents and children in order to best preserve their mental well-being during this pandemic. Public policies aimed at promoting resilience must incorporate targeted social policies to combat social inequalities.