This paper describes a quantitative study focused on two questions: (1) Can children understand and use a hierarchical domain structure to find particular instances of animals? (2) Can children construct search queries to conduct complex searches if sufficiently supported, both visually and conceptually? These two questions have been explored in the context of developing a digital library interface (called "QueryKids") for children ages 5-10 years old that visualizes the querying process and its results. The results of this study showed that children were able to search very efficiently, primarily using a "fewest-steps" strategy, with the QueryKids software prototype. In addition, children were able to construct search queries with a high degree of accuracy. Results are discussed in terms of the scaffolding support that QueryKids provides, and its effectiveness in helping children to search efficiently and construct complex search queries. Keywords: Children, information retrieval, digital libraries, empirical evaluation, education applications (Also cross-referenced as HCIL 2000-19) (Also cross-referenced as UMIACS-TR-2000-68)