Rumination and worry are common forms of perseverative cognition in children. Research has started to target perseverative cognitions in the everyday life of children, however, valid measurement instruments reliably capturing rumination and worry in children's daily life are still missing. We conducted two ambulatory assessment studies validating short scales suitable for the measurement of rumination and worry in children's daily life. Results of the first study (N = 110, 8-11 year-olds, 31 days, up to 4 daily measurements) supported a unidimensional structure of the rumination scale. Rumination was associated with negative affect (but not positive affect) on the within- and on the between-person level. On the between-person level, children who ruminated more showed poorer working memory performance. In the second study (N = 84, 8-10 year-olds, 21 days, up to 3 daily measurements), findings of Study 1 were largely replicated. Moreover, we established a unidimensional worry scale in Study 2 reliably capturing worry in children's daily life. Importantly, Study 2 showed that worry and rumination share common variance but can be differentiated in children. On the within-person level, higher levels of worry were associated with higher levels of negative affect and lower levels of positive affect. On the between-person level, worry was associated with higher levels of negative affect and lower working memory performance. Altogether, findings of both studies demonstrated that the short scales had excellent psychometric properties suggesting that they are helpful tools for the assessment of rumination and worry in children's daily life. Public Significance Statement The present work evaluated two short scales measuring rumination and worry in children's daily life. Results supported the validity and reliability of the two short scales and showed that fluctuations of rumination and worry are related but distinct phenomena in children. These measurement instruments provide valuable tools for research on children's experiences in everyday life. They might also be useful in clinical studies that track intervention-related changes in worry and rumination.