Abstract: Families with disabled children run a great risk of being poor. Although policies providing poor families with financial benefits should be effective in reducing poverty, the actual effectiveness is often jeopardized by the issue of non‐take up (NTU). Yet, NTU of benefits aimed at disabled children is for the most part uncharted territory. In this article, we fill this gap using a mixed‐methods approach to (a) estimate the magnitude and characteristics of NTU in the Belgian supplemental child benefit by drawing on a large‐scale administrative dataset on childhood disabilities and (b) explore the determinants of NTU by means of semistructured interviews with experts and parents. We estimate an NTU rate of at least 10%, a substantial figure given that the benefit is not income tested. This mainly concerns children with less visible disabilities (autism spectrum disorder and other intellectual and psychological disorders) and results from insufficient information provision about the benefit's existence and eligibility criteria; process costs, for instance, the complexity of the procedure; and the way the scale to assess a child's disability is constructed.