EEG in neonates and children allows for monitoring of the functional dynamics of the developing brain in real time. This is especially important in populations following atypical developmental trajectories, including preterm and acutely ill infants, those with seizures, and children with wider neurodevelopmental conditions. In this chapter, we focus on how EEG can shed light on paediatric sleep, and somatosensory, pain, and interoceptive processing. In particular, we emphasise its value in assessing these parameters naturalistically. The chapter is organised into (i) neonatal, and then (ii) paediatric applications. In the first part of the chapter, we illustrate how brain-body interactions during neonatal sleep can be assessed by integrating EEG with other physiological time series. We then describe how neonatal sensory cortical processing of meaningful, realistic inputs—e.g. feeding, self-generated movements—can be evaluated. In the second part, we focus on the use of EEG in children and young adults. We first examine how EEG can be used to assess disrupted sleep architecture. We then appraise its current capabilities in evaluating sensory states, namely chronic pain and interoceptive awareness. Throughout, we highlight the growing potential and feasibility of community-based settings, and more naturalistic paradigms, to improve the ecological validity of EEG assessments. Finally, we draw together the strands of both sections and discuss future directions for developmental EEG research, particularly its translational promise in complex paediatric populations.