Young children struggle more with mapping novel words onto relational referents (e.g., verbs) compared to non-relational referents (e.g., nouns). We present further evidence for this notion by investigating children's extensions of noun-noun compounds, which map onto combinations of non-relational referents, i.e., objects (e.g., "baby" and "bottle" for "baby bottle"), and relations (e.g., a "bottle" FOR "babies"). We tested two- to five-year-olds' and adults' generalisations of novel compounds composed of novel (e.g., "kig donka") or familiar (e.g., "star hat") nouns that were combined by one of two relations (e.g., "donka" that has a "kig" attached (=attachment relation) versus "donka" that stores a "kig" (=function relation)). Participants chose between a relational (shared relation) and a non-relational (same colour) match. Results showed a developmental shift from encoding non-relational aspects (colour) towards relations of compound referents, supporting the challenge of relational word referents. Also, attachment relations were more frequently encoded than function relations.