In 2020, the novel coronavirus SARS-CoV-2, and the resulting highly infectious disease COVID-19 led to restrictions based on the principal of social distancing to curb the spread of the virus among the population and to prevent an overload of health system capacities. These restrictions changed the daily lives of young children and parents dramatically. In a German questionnaire study, we aimed to investigate the impact of the COVID-19 pandemic on the magnitude of stress in parent-child systems and on social-emotional child development. Our sample consisted of 90 (39 male, 51 female) children (M = 17.2 months, SD = 9.7 months) aged 7=12 months (n = 38), 13-24 months (n = 31) and 25-38 months (n = 21). Parental stress was measured using the German version of the Parenting Stress Index, namely Eltern-Belastungs-Inventar. Additionally, social-emotional child development was measured using the Social-Emotional Questionnaire of the Bayley-III. Our findings show that parents experienced more stress during the COVID-19 pandemic in Germany compared to norms. Parental perceived stress was higher in parents of older children than younger ones. Interestingly, social-emotional child behavior scores significantly decreased with children's increasing age. Moreover, higher parental stress was associated with lower values of social-emotional child behavior. Our findings provide important novel data on parental perceived stress and social-emotional child development during the COVID-19 pandemic. However, further research investigating the long-term consequences of the pandemic is needed.