The present research focuses on the measurement of a positive emotion of great relevance for the field of mental health and education. The main purpose was to develop and validate two independent scales to assess serenity in children: a self-report instrument and an external observer report. In Study 1, we evaluated the psychometric properties of a self-report instrument. The participants of this study were 613 children aged between 9 and 12 years old (44.70 % boys and 55.30 % girls), that attended public schools in the city of Buenos Aires, Argentina. The scale consists of 11 items with three response options on a Likert scale ( Yes , Sometimes , and No ), and presents a comprehensible language for school-age children. It includes both general statements ( e. g. , “I am relaxed most of the time”, “Most days I feel at peace”), as well as statements that refer to the search for serenity in threatening or stressful situations ( e. g. , “When someone bothers me I try to stay calm”, “When things go wrong I try to calm down”). Study 2 was carried out to assess the psychometric properties of a serenity scale that takes into account the perspective of an external observer close to the child, such as parents or educators. This scale is appropriate for obtaining information from young children without the ability to read and write, from four years old and up. In addition, due to its short extension, it is ideal to work in the school context, when the teacher is required to complete the information for large groups of students. The non-randomized sample was composed of 365 children (51.4 % boys and 48.6 % girls), between 4 and 12 years of age, that attended the initial levels ( M age = 4.60; SD = .50) and primary schools ( M age = 8.13; SD = 1.68), from the province of Entre Rios, Argentina. Teachers were responsible of answering the scale, in reference to each of their students. The report contains six items, with three response options on a Likert scale (Yes, More or less, and No). The items were formulated as questions that made reference to a threatening situation in which the child could exhibit or not the ability to calm down ( e. g. , “When the child encounters a problem, does he/she make an effort to stay calm?”, “Can the child stay calm even if he/she is in a noisy or exciting environment?”). In both cases, an instrumental study was carried out to analyze the items’ discriminative power, the internal consistency, the construct validity and the nomological validity of the scales. In both studies, the results indicated that the items have an adequate discriminant capacity, a high internal consistency, and are clearly grouped in a single general factor. They also provide evidence for nomological validity, since they correlate negatively and significantly with measures of aggression, hyperactivity and impulsivity. It is concluded that these new measures of serenity constitute an interesting contribution for the field of psychological evaluation, providing reliable and valid data on a positive emotion that has been scarcely studied in childhood. This study also revealed that serene children are less likely to show aggressive behaviors, such as hitting, kicking or insulting, when they relate to their peers; and that, in addition, they would show less hyperactivity and a smaller number of impulsive reactions. It would be advisable then that schools and parents could have a more active role in the teaching of exercises to encourage inner calm, in order to help their children to respond more responsively to the stressful situations in their environment. https://doi.org/10.16888/interd.2021.38.2.3