Metacognition is important for monitoring and regulation of cognitive processes, decision making, problem-solving and learning. Despite the widespread interest in metacognition, measuring metacognition in children poses a significant challenge. Some qualitative and observational measures exist, but they are restricted by the number of components they measure and the sampling size. Some meta-cognition tasks of memory have been developed for children, but these only measure a narrow range of skills involved in metacognition. A novel metacognitive problem-solving task, previously developed by the lab, provides scalable means to holistically measure metacognition. The thesis, developed a new coding scheme, recoded the data and studied the reliability and validity of the novel task by comparing it with demographic variables known to be associated with metacognition and a metamemory task. The results indicate the novel task is reliable and valid. It operationalizes metacognitive measures similarly to a classical metamemory task, suggesting that the new task could be a bridge between existing measures of metacognition in children and adults. The thesis uses the novel task to then explore other broader questions with 182, 8- to 11-year-old students, pertaining to cognitive levels in low socioeconomic status, ethnic-minority students, domain-generality/specificity of metacognition and the association between metacognition and executive-functions. The results indicate that low socioeconomic status, ethnic-minority students have poor cognitive levels and low amounts of cognitive development across the various age groups. The results also suggest metacognitive components to be domain-general in nature and were tapped into by the novel metacognitive problem-solving, metamemory and a complex executive-function tasks. The results provide further evidence for association between metacognition and executive-functions.