ScratchJr is a block-based language intended for young children aged 5 to 7 years old. The language and its app were developed using a learner-centered approach to make it usable and understandable to children. Obviously, Early Childhood teachers using ScratchJr must develop, as part of their content knowledge, an understanding of both the language and programming basics. Unfortunately, the materials made available by the constructors of ScratchJr were not envisaged for teacher development but for teacher instruction. Based on our concern with providing teachers a deep understanding of block-based programming, we present in the paper a study about teaching ScratchJr to pre-service teachers. It involved two groups, one trained with the materials developed by the ScratchJr creators and another one with materials that explain the run-time semantics of the different blocks of ScratchJr. The performance of both groups was compared with six questions extracted from the global exam of the course. The results show that pre-service teachers instructed with the novel approach performed better in predictive exercises or exercises focused on block understanding but performed worse in exercises consisting in translating a written behavior into scripts.