The interest in studying idioms arises from the features that characterize these expressions: (a) often they are ambiguous sentences that have both a literal and a figurative meaning which significantly differ from each other (e.g. consider the literal and figurative meaning of the idiom 'to break the ice'); (b) they are conventionalized expressions, shared by the members of the linguistic community and are represented in the mental lexicon; and (c) as such they have to be acquired by children as part of the linguistic repertoire (Levorato & Cacciari, 1992; Levorato, 1993). Despite the fact that these expressions are very common both in oral and written language (for details, see Nippold & Rudzinski, 1993), children's ability to comprehend and use them develops during their school years, when they are already competent speakers, whereas children younger than seven tend to interpret them literally. Idiom acquisition, then, is a long lasting and complex process: it is presumably for this reason that the study of the processes underlying idiom acquisition in children, which began in the 1970s (Lodge & Leach,1975), is still a subject which arouses interest.

Main results to date 
Idiom acquisition is based on a variety of abilities, different in nature and complexity, involving cognitive, linguistic and pragmatic competence. 
Various factors affecting developmental changes in idiom comprehension have been identified, the most important of which are familiarity, semantic analyzability and context.
Whereas until the 1990s studies focused primarily on factors which favour idiom comprehension, more recent research has turned its attention to individual differences and the relationship between idiom comprehension and other linguistic abilities. The relationship between idiom comprehension and text comprehension is considered particularly interesting in the light of the importance of processing of the linguistic context and it was demonstrated in school aged children, from the first grade to the late childhood. Recently this relationship was found also in individuals with Down syndrome. 
Finally, there is some evidence that the relationship between text and idiom comprehension might be causal: in a longitudinal study on less skilled comprehenders it was demonstrated that an improvement in text comprehension was paralleled by an improvement in idiom comprehension.

Selected Publications

Roch & Levorato (2010). Idiom understanding in children and adolescents with down syndrome: The role of text comprehension skills. Applied Psycholonguistics, 31 (3), 531-530. I.F. 2.13

Levorato, M.C., Roch, M. Nesi, B. (2007). Idiom understanding in first graders: a follow-up study on less skilled comprehenders. Journal of Child Language, 34 (3), 473-494.

Nesi, B., Levorato, M.C., Roch, M. & Cacciari, C. (2006). To break the...embarrassment: Text Comprehension Skills and Figurative Competence in skilled and less-skilled text comprehenders. European Psychologist, 11 (2), 128-136.

Levorato M.C., Nesi B. Cacciari C. (2004). Reading comprehension and understanding idioms: a developmental study. Brain and Language, 202, 4-16.