The present study investigates children's syntactic and pragmatic processing when specifying referents presented in short video clips. Within Relevance theory, the assumption of 'optimal relevance' implies that utterances are intended to involve the least processing effort on the part of the listener. In the present context, lexically specified NPs are assumed to be more in line with optimal relevance than pronouns. Subjects were 48 normally developing children aged 3;4-8;10 and 30 SLI children aged 5;1-8;9, divided into a low and a normal MLU group. Children's responses were coded according to levels of pragmatic processing and syntactic positions. Normally developing children's referent specifications were found to be increasingly relevant with increasing age. Differences between SLI and normal children were only found for the low MLU group with SLI who used fewer pronouns than the younger children, thereby showing that syntactic limitations alone cannot account for children's specification of referents.