This thesis investigates lexical development in early sequential bilinguals based on cross-linguistic data from thirty-eight Polish-English speaking children and twenty-four age-matched Polish monolinguals. Using a variety of methods, we examine the nature of lexical development, and potential shifts in relative language dominance in child heritage speakers of Polish across the primary school years. The aim of this study is to evaluate early changes in heritage lexical acquisition, their characteristics and the age at which they occur, and to determine whether they form a pattern that precedes and leads to a switch in language dominance in middle childhood. A new test, the child HALA, is introduced to measure shifts in relative language strength by comparing lexical accuracy and access between two languages. This test has been designed specifically for use with children, and is based on the HALA psycholinguistic tool (O'Grady et al. 2009). The results show that child heritage speakers of Polish display overall levels of noun acquisition comparable to monolinguals. However, we note that they demonstrate slower language access, and a reduced range of lower-frequency nouns when compared with monolinguals. Their relative language dominance shifts from the initially stronger Polish to the more dominant English between the age of eight and eleven. On the basis of the findings we establish a timeline of of changes in heritage lexical acquisition from the onset of continuous exposure to the majority language to the switch in language dominance. We also find that the Child HALA test produces reliable results across age groups and languages when compared with other methods of measuring lexical proficiency, and therefore proves to be a valid method in assessing language strength and maintenance in children.