Background: As a result of the COVID-19 pandemic, governments have implemented preventive measures towards reducing infections. These conditions can affect the mental health of children and adolescents; however, this has not yet been fully explored. Objective: The aim of the study was to analyze changes in symptomatology and positive emotions in Argentine children and adolescents since the onset of isolation, based on parent/caregiver report. We analyzed differences based on gender, age, socioeconomic status (SS) and containment measure (and their interactions); their associations with symptomatology and positive affect of parents/caregivers; and the moderating effects of sociodemographic factors on these associations. Method: A total of 1205 caregivers responded to a survey regarding the mental health of children and adolescents under their care. They also completed a set of anxiety, depression, and affect measures about themselves. Results: A considerable proportion of parents/caregivers perceived changes in their children's and adolescents' mental health compared to before the pandemic. Increased levels of anxiety-depression, aggression-irritability, impulsivity-inattention, and dependence-withdrawal were reported, as well as alterations in sleeping and eating habits, and a reduction in positive affect. Differences were observed according to their age and containment measure. Finally, we found correlations between parents/caregivers' symptomatology and that reported about their children or adolescents. Gender, age and SS moderated some of these relationships. Conclusions: Continued monitoring of child and adolescent mental health is a fundamental necessity. We recommend the implementation of early intervention strategies to prevent the escalation of serious mental health problems, particularly in those groups that have been most adversely affected since the onset of the pandemic.