Background: Parental support and participation in physical activity (PA) with children and parents’ acting as a role model for less sedentary behaviors (SB) are critical factors for children’s healthier lifestyle. The purpose of the study was to assess the relationship between mothers’ enjoyment and participants’ sedentary behavior (SB) and physical activity (PA) as a secondary analysis of a randomized controlled trial (RCT) using data from Moving Sound RCT in the Pirkanmaa area of Finland. Methods: The participants were 108 mother–child dyads (child age 5–7 years) who completed the eight-week exercise intervention using a movement-to-music video program in their homes. Mothers’ enjoyment was examined using a modified version of the enjoyment in sport questionnaire. The proportion of SB, standing, light PA, moderate-to-vigorous PA, and Total PA were derived from accelerometers at baseline and during the final week of the intervention. Analyses were performed using linear mixed-effect models for (1) intervention and control groups, (2) groups based on mothers’ enjoyment. Results: The results highlighted that mothers’ enjoyment of exercise with their children was overall high. Although there was no difference between the intervention and control groups, mothers in the intervention group increased their enjoyment during the intervention (p = 0.007). With mothers’ higher enjoyment at baseline, children’s light PA increased (p < 0.001), and with mothers’ lower enjoyment, children’s SB increased (p = 0.010). Further, if mothers’ enjoyment decreased during the study, their own LPA increased (p = 0.049), and their children’s SB increased (p = 0.013). If mothers’ enjoyment remained stable, children’s light PA (p = 0.002) and Total PA (p = 0.034) increased. Conclusions: In this RCT, no differences were found between the intervention and control groups or groups based on mothers’ enjoyment, possibly due to the low power of the study. However, mothers’ enjoyment of exercise with their children increased within the intervention group, and mothers’ enjoyment influenced children’s SB and PA. For future studies, it would be essential to focus on children’s enjoyment and factors behind the behavior change.