Rules for interpreting empty category (EC) subjects of complement clauses vary crosslinguistically across structural and lexical dimensions. In adult Greek, a distinction is made between the verbs meaning WANT and TRY, the former but not the latter permitting the EC subject of its subjunctive complement to refer outside the sentence. The EC is pro for WANT and PRO for TRY. In adult Spanish, both the verbs meaning WANT and TRY require the EC subject (pro) to refer outside when the complement is in the subjunctive, and require the EC subject (PRO) to refer to the main clause subject when the complement is in the infinitive. Twenty-three Greek-speaking four- to five-year-olds and 10 adults, 29 Spanish-speaking four- to five-year-olds, 18 six- to seven-year-olds and eight adults took part in act-out experiments. The results indicate an awareness of language-particular distinctions governing the interpretation of EC complement subjects. However, child speakers of both languages experience difficulty in giving sentence external reference, leading to error in the case of subjunctive sentences for Spanish-speaking children. We argue that the data overall is most compatible with children having access to the empty category PRO by age four, and that failure to give external reference of an EC when required can plausibly be treated as performance error. A picture verification task produced less clear results, but points to the need for data from younger children to establish whether there is an early stage in which lexical semantics dominates children's interpretation of ECs.