Local authorities are increasingly required to measure and monitor the well-being of children. One key aspect of ensuring that these data on child well-being are robust is that they form an accurate and representative picture of that child population. Rather than being only the purview of researchers, the act of gathering outcome data now falls within the remit of policymakers and practitioners. This article therefore describes the principles and importance of solid sampling methodologies to this audience and also considers the practical reasons as to why policymakers and practitioners may require larger samples than may be necessary for a representative picture of child well-being. These ideas are illustrated by one local authority in the UK's approach to measuring child well-being, using representative sampling techniques and the subsequent expansion of this sample. The consequences of expanding the sample size are illustrated by examining the impact upon key findings related to child well-being. The potential benefits and limitations of larger samples are discussed, as are some practical solutions to the apparent impasse.