The dynamic nature of multilingual families and their language policies has been touched upon by numerous studies. Adding to the field, the present study assesses the stability of family language policy in a standardised and quantitative manner. To this end, a linguistically heterogenous sample consisting of 488 multilingual families raising young children in Belgium's Flemish Community was surveyed twice, eighteen months apart. Based on the collected longitudinal survey data, the present study offers statistically verifiable evidence for the (partially) dynamic character of family language policy. Firstly, parental beliefs in a multilingual advantage were strengthened and a change was found in language-specific beliefs regarding children's language acquisition. Secondly, the families' practices demonstrate a shift towards the Dutch institutional language, particularly in parental language use when communicating with each other and with their child, and in the child's overall exposure. The observed shift in practices and beliefs underscores not only parents' continuous assessment of their children's linguistic needs and development, but also the societal environment influencing this assessment and adjustment. However, while significant changes in language beliefs and practices were uncovered, the more conscious and explicit component of language management proved stable across time, corroborating the independent character of the three family language policy components.