Growing up bilingual has a remarkable influence on language processing and brain development in children, yet a systematic neurobiological overview remains elusive. The current review addresses this gap by collecting, summarising, and critically examining findings on the effect of bilingual experience(s) on brain development and language processing in infants, toddlers, and children, with a focus on neurobiological methodologies. In this review, we scrutinise the developmental (neural) trajectories of early and late bilingual children, based on 94 peer-reviewed articles. In early-immersive bilinguals, dual-language input influences various aspects of early language development and processing. For instance, they initially demonstrate delayed perceptual narrowing compared to monolinguals. Subsequently, during semantic and syntactic processing, bilinguals often demonstrate behavioural performance comparable to monolinguals; however, neuroimaging studies consistently reveal additional recruitment of executive function regions beyond the canonical language network. Furthermore, early bilinguals and monolinguals exhibit numerous differences in brain structure and function, indicating ongoing neural refinement and potentially more efficient language processing. Regarding second-language learning children (L2 learners), the focus is on the interaction between neural adaptation and language input. As input and proficiency increase, L2 learners demonstrate more mature ERP responses and greater activation within language-specific brain regions. In sum, this review highlights the dynamic neuroplasticity and distinct neural mechanisms linked to the bilingual experience from infancy onward. Finally, this review provides a comprehensive literature overview of the developing bilingual brain, emphasising the substantial impact of bilingualism on both language-specific and general cognitive networks, and highlighting possible future directions.