Do children's representations of complex words in their mental lexicon have an internal structure, with the stem as a separate unit? De Jong et al (2000) found that adults recognize words with a large Family Size, i.e. words occurring in many derivations and compounds, faster than equally frequent words with a small Family Size. This result is an indication that the occurrence of a stem in complex words facilitates the recognition of this stem. This article investigates whether the Family-Size-effect extends to children's reaction times as well. Using a lexical decision task, the effect was observed in 9-10 year old children (N=69) in Dutch. A large vocabulary and good reading ability shortened general reaction times, but had no influence on the difference between items with a small or large Family Size. Monolingual and bilingual children performed similarly on this task.