Impact belief is the conviction that parents have that they can affect their children’s language development (De Houwer, 1999). This paper investigates how parents’ impact belief is shaped and how it transpires into language management which supports the bilingual and biliterate development of children in exogamous families. Interviews with eight English-speaking parents raising English-Japanese bilingual children in Tokyo, Japan were analyzed using the constructive grounded approach (Charmaz, 2014). The results revealed that the parents’ impact belief was influenced by their individual experiences, the support of their Japanese spouses, and peer influence. Specifically, it was positively affected by other parents with older bilingual children. The parents’ impact belief was also strengthened by their involvement in ‘communities of practice,’ i.e., English playgroup and weekend school. Their strong impact belief led to language management efforts which included their insistence on their children speaking English and the regular practice of home literacy activities.