This study investigates the acquisition of the passive structure from the point of view of a conciliatory approach between computational metrics and processing demands. The formal analyses from Generative Linguistic Theory (Boeckx, 1998; Collins, 2005) are revisited and made compatible to the distinctions proposed by the Integrated Model of on-line computation (Corrêa AND Augusto, 2007; 2011) -Modelo Integrado da Computação on-line, considering the peculiarities of on-line processing. Our hypothesis is that the structural complexity associated with an eventive passive would imply the existence of measurable computational cost movement, which would be responsible for the difficulty children have on verbal passives. The absence of the relevant type of movement in adjectival passive constructions with copular characteristics would explain why children are initially capable of dealing with short agentive passives. We assume that, in Portuguese, children do not attend to the differences auxiliary selection implies in the target grammar. Three psycholinguistic experiments were conducted, in order to investigate the hypotheses proposed. The first two experiments show some difficulty on every verbal passive in Portuguese (except for short agentive passives) and highlight the relevance of aspectual and semantic features allowing a simpler derivation to be licensed in the logical form. The last experiment evaluated children´s ability to distinguish the different auxiliaries involved on the derivation of short passive sentences. The results are compatible with the hypotheses and suggest that children would take adjectival constructions into account in achieving full acquisition of verbal passives.