The term ‘double burden of malnutrition’ is usually interpreted in terms of the physical status of children: stunted and wasted children on the one hand and overweight/obese children on the other. There is a third category of malnutrition that can occur at either end of the anthropometric spectrum or, indeed, in children whose physical size may be close to ideal. This third type is most commonly articulated with the phrase ‘hidden hunger’ and is often illustrated by micronutrient deficiencies; thus, we refer to it here as ‘undernutrition’. As understanding of such issues advances, we realise that there is a myriad of factors that may be influencing a child’s road to nutritional health. In this BMC Medicine article collection we consider these influences and the impact they have, such as: the state of the child’s environment; the effect this has on their risk of, and responses to, infection and on their gut; the consequences of poor nutrition on cognition and brain development; the key drivers of the obesity epidemic across the globe; and how undernourishment can affect a child’s body composition. This collection showcases recent advances in the field, but likewise highlights ongoing challenges in the battle to achieve adequate nutrition for children across the globe.