Just-in-time adaptive interventions (JITAIs) are a promising technology-based approach for health behavior change. This examination aimed to evaluate whether a JITAI after a period of inactivity can enhance physical activity in the subsequent hour depending on whether the JITAI has been answered ("engaged" condition) compared to when the trigger was not answered ("not engaged" condition). Data of the three-week intervention period of the SMARTFAMILY2.0 trial was used for analysis. A total of 80 participants (n = 47 adults, 23 female; n = 33 children, 15 female) with 907 JITAI triggers were included in this examination. A JITAI was sent when the participant has been inactive for at least 60 minutes as indicated by accelerometry. Two multilevel models were calculated for metabolic equivalents (MET) and step count with measurements (level 1) nested in participants (level 2) under consideration of the covariates weekday/weekend, time of the day, and adult/child. Results indicated significantly higher MET (β = 0.08, p = .014) and step (β = 0.08, p = .022) counts in the subsequent hour for the engaged condition compared to the not engaged condition within-persons (level 1). Engagement with the JITAI implemented in the SMARTFAMILY2.0 trial yielded promising results concerning physical activity enhancement in the subsequent hour. Here, the inclusion of further constraining factors like the availability of the participant or the inclusion of affective and contextual variables into the design of a JITAI might enhance the engagement in future studies.