The term â€˜well-beingâ€™ is increasingly used in Early Childhood Education & Care (hereafter ECEC) policy contexts as a desirable outcome for children. In large part, this outcome appears to be predicated on children achieving stage specific educational goals and being â€˜ready for schoolâ€™. However, in spite of its ubiquitous use, child well-being remains under-theorised, thereby contributing to implicit understandings within policy arenas. Such understandings may not be reflected by the recipients of, or those charged with implementing this policy. This study therefore set out to explore: first, how ECEC policy currently theorises child well-being; second, how parents, early years educators and young children themselves conceptualise child (their) well-being; and third, the implications of these understandings for the policies and practices of working optimally together in the interests of children in low-income areas. To do so, a theoretical framework of child well-being was developed as the studyâ€™s framing and analytical tool. This framework was developed by exploring current well-being theories and considering how they are influenced by and contribute to prevailing social constructions of young children. The concepts privileged by each of the theories were used to critique ECEC policy and its implications for young childrenâ€™s well-being. These concepts were, in turn, applied to perspectives of the three under-represented groups in ECEC policy formation. These views were generated in a small-scale qualitative study (conducted between July 2016 â€“ June 2017) which involved 18 children aged two - four years and seven each of parents and early years educators in a low-income area in England. Semi-structured interviews and the Mosaic Approach were used to generate data with adults and children respectively. The findings suggest that ECEC policy is narrowly conceived. Its measurement practices and curriculum goals, in particular, may undermine, while at the same time characterising itself as espousing, practice supportive of young childrenâ€™s well-being. The three respondent groups had broader conceptualisations of child well-being. They understood well-being to be inter-dependent with that of others. In this sense well-being was seen to be not only under-theorised, but under-socialised and de-historicised. Consequently, an integrative approach to well-being is proposed which neither privileges nor abstracts children from their social and material contexts. A reconceptualisation of childhood away from prevailing deficit social constructions, a recalibration of ECEC policy and practice to be more responsive to childrenâ€™s wider contexts, and recognition of the broader social and material factors influencing m/others and their shared environments would support all their well-beings. The thesis contributes to knowledge by developing a theoretical framework, which provides a more holistic conceptualisation of young childrenâ€™s well-being in ECEC generally and for those in low-income areas in particular. It is also the first study, to my knowledge, to report the subjective well-being of children under the age of five years.