Background: There is evidence that reward responsiveness and optimism are associated with mental and social functioning in adolescence and adulthood, but it is unknown if this is also the case for young children. Part of the reason for this gap in the literature is that the instruments that are used to assess reward responsiveness and optimism in adolescents and adults are usually not suitable for young children. Objective: Two behavioral tasks to assess reward learning, a questionnaire on reward responsiveness, and a questionnaire on optimism/pessimism will be tested on their feasibility and reliability in children aged 6-7. Depending on their feasibility and reliability, these instruments will also be used to investigate if reward responsiveness and optimism are associated with mental and social functioning in young children. Methods: For this cross-sectional pilot study, we adapted a number of tasks and questionnaires to the needs of 6-7-year-old children, by simplification of items, oral rather than written assessment, and reducing the number of conditions and items. We will approach teachers and, with their help, aim to include 70 children aged 6-7 to assess the feasibility and reliability of the tasks and questionnaires. Feasibility measures that will be reported are the proportion of children completing the task/questionnaire, the proportion of children that were able to explain the instructions in their own words to the researcher, and the proportion of children that correctly answered the control questions. The reliability of the scales will be assessed by computing Cronbach α and item-total score correlations and the reliability of the tasks by correlations between different consecutive blocks of trials. Ethics approval was obtained from the Ethics Committee of the Department of Pedagogy and Educational Sciences. Results: Data collection was originally planned in March and April 2020, but has been postponed due to Corona virus regulations. We expect to collect the data in the first half of 2021. The findings will be disseminated in preprints and peer-reviewed publications. Conclusions: The development of feasible and reliable instruments for assessing reward responsiveness and optimism in young children is expected to benefit future research on underlying mechanisms of mental and social functioning in young children. If the instruments assessed in this study are usable with young children, it would be particularly interesting to include them in cohort studies because this would enable investigating not only concurrent associations, but also prospective associations between reward responsiveness and optimism early in life and mental and social functioning later in life. If, as we hypothesize, reward responsiveness and optimism are not only associated with (prospective) mental and social functioning in adults and adolescents but also in young children, this could provide a way of identifying vulnerable children already at an early stage.