In this particular situation, trisomy of chromosome 21 is the genetic abnormality most frequently associated with cognitive impairment. Down syndrome results from this genetic abnormality. It impacts development in both the physical and cognitive domains. Memory, executive function, the language domain are all cognitive domains that are affected by cognitive impairment. Cognitive impairment also affects the ability to make decisions and impacts daily functioning. This study seeks to determine the prevalence of cognitive impairment among children with Down Syndrome and identify particular impairment domains, including memory, executive function, and language. This study presents the creation of an informant-rated assessment, the Cognitive Scale for Down Syndrome (CS-DS), which emphasizes daily activities associated with executive function, memory, and language and is appropriate for evaluating these skills in children with Down syndrome. A quantitative cross-sectional study using a purposive sample method included 63 children from Dhaka city. Data was gathered utilizing the Cognitive Scale for Down Syndrome (CS-DS). The data was processed utilizing version 25.0 of the Statistical Package for Social Sciences (SPSS) (IBM SPSS Statistics 25). Sixty-three children with Down syndrome were selected, including 60.3% males (38) and 39.7% females (25). The highest proportion of children (36.5%) were 15 to 18 years old. This study compared the scores of Severe Cognitive Impairment (0-61) and Mild Cognitive Impairment (62-122) on the CS-DS questionnaire, indicating that most children with Down Syndrome had restricted cognition. According to CS-DS, all children in this study demonstrated some cognitive impairment. Within the assembly of individuals, According to CS-DS, 66.7% exhibit severe mental impairment, whereas 33.3% demonstrate mild cognitive impairment. Individuals aged 6 to 8 exhibited more significant impairment in executive function, memory, and language domains. Boys with Down syndrome exhibited more difficulties in executive function, memory, and language domains compared to girls. According to CS-DS, all patients in this study showed some cognitive impairment. The survey's findings offer essential insights for ongoing study and research. Keywords: Down Syndrome, Cognitive Impairment, Executive Function, Memory & Language domain.