School closure due to coronavirus disease 2019 (COVID-19) pushed children across ages and nationalities into a state of mental health crisis. In Japan, children between the ages of 6 and 18 were ordered to stay at home and observe social distancing for several months. This study is aimed at investigating the effects of quarantine due to COVID-19 on children belonging to different developmental stages in life. Data were collected from mothers of typically developing children aged between 6 and 18 years. The differences in psychological and behavioral changes following school closure during the COVID-19 pandemic were explored. A total of 535 children, including 145 students in lower grades of elementary school (6–9 years), 124 students in higher grades of elementary school (9–12 years), 132 students in junior high school (12–15 years), and 134 students in high school (15–18 years), were recruited. Children in lower grades of elementary school (lower grades group) gained significantly lower understanding about COVID-19 and the necessity of COVID-19 restrictions than children in the other groups. Moreover, they had more psychological problems: they easily cried and complained, were unable to keep calm, and were dependent on parents and family members. Changes in sleep patterns were more prevalent in junior and senior high school students. We concluded that mental health care should be provided based on the growth period of each child not only during school closure but also after school reopening.