Many children are introduced to LEGO® bricks for the first time at a young age. LEGO® models are usually built according to a set of building instructions, each instruction step being printed onto paper as an orthographic projection. However, as children’s spatial reasoning skills are yet to be fully developed, they tend to misinterpret positions and shapes from these types of instructions, resulting in suboptimal performance in LEGO® construction tasks. With Augmented Reality (AR), users can perceive and interact with three-dimensional, virtual content as if it was present in the real world, eliminating the need to convert two-dimensional instructions into three-dimensional models. In this work, an AR-Assisted LEGO® Building Instruction (ARALBI) application was developed, providing three-dimensional reference LEGO® models. ARALBI was specifically designed to be used by preschool children, with them constructing the real model step by step next to the virtual model. It was tested against typical paper-building instructions in a user study based on the General Assembly Task Model (GATM). Using AR instructions, preschool children performed better in terms of effectiveness (i.e. made fewer mistakes) but not efficiency (i.e. required more time to finish assembling the model) when compared to paper instructions. The results of this study indicated that preschool children, who never used AR technology before, enjoyed the use of the AR application and preferred it over paper-building instructions.