Large portion sizes can make children overeat, alter their self-regulation abilities and induce weight gain. However, little is known about how parents determine portion sizes for their children. Using semi-structured interviews with 5 fathers and 32 mothers of pre-schoolers, this study examined French parents’ food portioning practices. The division of responsibility between parent and child in deciding portion sizes was explored, as well as the influencing factors and possible sources of information. Parents described a wide range of practices. For most, determining portion sizes is an intuitive action that depends on habits and mainly arises from experiences with feeding their child and his/her appetitive traits. Few parents grant autonomy to their child for portioning and serving food, especially for the first serving. Many influencing factors were identified, including child-related (e.g., appetite, food preferences), parent-related (e.g., avoiding food waste), and external factors (e.g., influence of siblings, French food culture). Most parents do not search for information/recommendations to guide their practices. Stimulating optimal self-regulation of eating in children is important and parents can play a crucial role in this. This study identified barriers and facilitators to guide parents in providing appropriate portion sizes and help include children in this decision process.