Italian relative clauses like "Il bambino che bacia la mamma" 'the child that kisses the mom' are ambiguous between a subject reading and an object reading with postverbal subject. However, the latter is scarcely accessible for word order and theory-internal considerations. This study aims at investigating the role of semantic (im)plausibility in processing these ambiguous constructions. Italian children's (7;01-10;00 years old) and adults' (21;08-31;02 y.o.) comprehension is tested through a picture selection task. The test sentences contain lexical verbs whose interpretation can be modulated by encyclopedic knowledge (e.g., to "spoon-feed"). In the ambiguous sentence "Il bambino che imbocca la mamma" 'the child that spoon-feeds the mom,' the object reading is more plausible: Moms rather than children are expected agents of the spoon-feeding. Nonetheless, word order and morphosyntactic and prosodic cues prompt the subject interpretation. Results indicate that semantic plausibility cues alone are not robust enough to discard the subject reading. However, adults are sensitive to these cues, which can modulate their comprehension of ambiguous relatives. Conversely, children are unable to exploit encyclopedic knowledge in sentence processing. This can be explained with children's reluctance to integrate contextual and encyclopedic semantic cues during processing, and with their limited processing resources, which could constrain their capacity of sentence reanalysis.