The purpose of this study is to understand the developmental and mental health status of children during post-COVID-19. For this study, 456 children at daycare centers in 25 cities in Seoul were selected as subjects for the study, and the infant evaluation was conducted from June to September 2022. We evaluated children using screening tools that measure language (K-SNAP/REVT) and cognitive (VMI) development levels. In addition, a 20-minute play evaluation session (semi-structured) was recorded. Emotional and behavioral problems were measured by both parents (CBCL) and teachers (TRF). In 290 children who showed any risk in any of these screening measures, a child psychiatrist and a developmental psychologist examined the play assessment recordings and performed clinical diagnoses based on DC 0-5 and DSM-5. As a result, 290 (64%) children were selected as showing risk in screening measurements. Of those, 254 were examined by clinicians, and a total of 219 (48%) children were classified as clinical status of the 152 in the diagnostic group and 68 in the subclinical diagnostic group. These results announce the need for early diagnosis of children suffering from developmental and mental health difficulties during the post-COVID-19 period and are expected to serve as basic data for preparing intervention methods to promote the developmental and therapeutic promotion of children.