Research on the structure of children’s affect is limited. It is possible that children’s perception of their own affect might be less differentiated than that of adults. Support for the 2-factor model of positive and negative affect and the pleasure-arousal model suggests that children in middle childhood can distinguish positive and negative affect as well as valence and arousal. Whether children are able to differentiate further aspects of affect, as proposed by the 3-dimensional model of affect (good-bad mood, alertness-tiredness, calmness-tension), is an unresolved issue. The aim of our study was the comparison of these 3 affect models to establish how differentiated children experience their affect and which model best describes affect in children. We examined affect structures on the between- and within-person level, acknowledging that affect varies across time and that no valid interpretation of either level is feasible if both are confounded. For this purpose, 214 children (age 8–11 years) answered affect items once a day for 5 consecutive days on smartphones. We tested all affect models by means of 2-level confirmatory factor analysis. Although all affect models had an acceptable fit, the 3-dimensional model best described affect in children on both the within- and between-person level. Thus, children in middle childhood can already describe affect in a differentiated way. Also, affect structures were similar on the within- and between-person level. We conclude that in order to acquire a thorough picture of children’s affect, measures for children should include items of all 3 affect dimensions.