Since the COVID-19 pandemic began, children have presented with increased psychiatric symptoms. Little research has been done regarding early childhood mental health, particularly those from vulnerable socioeconomic contexts who are exposed to adversity. We aimed to assess mental health and the impacts of the pandemic on this population. A survey and the Strength and Difficulties Questionnaire were administered to the caregivers of preschoolers who were enrolled in a food-assistance programme. The participants were 807 preschoolers from the City of Buenos Aires, Argentina, which was ranked among the cities with the longest lockdown. Around 39% of preschoolers were classified as having a 'possible/probable' psychiatric disorder. Externalizing problems were predominant. Most caregivers (82.78%) reported increased psychological symptoms during the lockdown, mostly externalizing problems. Caregivers' burdens were associated with the severity, duration, and exacerbation of their child's symptoms. Further research should continue to monitor preschoolers' well-being, with the goal of preventing future problems.