Parent–child separation threatens the perceived mutual trust in parent–child relationships and child developmental outcomes. Even though pioneering researchers have examined the development of children living apart from their parents, few studies have investigated the perceived mutual trust in the parent–child relationship and child developmental outcomes in boarding schools resulting from temporary parent–child separations. To determine perceived mutual trust, the content of conversations, feelings related to the use of mobile phones in parent–child conversations, and the effects thereof on child developmental outcomes, we adopted a cross-sectional survey of 1136 boarding school students recruited by a multi-stage stratified random-sampling approach from different districts in Sichuan, China. The data were analyzed with multiple regression analyses. The results of the present study support the hypothesis that a child’s perceived mutual trust is positively associated with their developmental outcomes, in addition to partially validating the moderating effect on the content of mobile phone conversations and feelings related to mobile phone conversations. This research provides empirical evidence regarding the influence of perceived parent–child mutual trust on boarding students and their developmental outcomes in the digital age.