Research on learning and instruction of science has shown that learning environments applied in preschool and primary school rarely makes use of structured learning materials in problem-based environments although these are decisive quality features for promoting conceptual change and scientific reasoning within early science learning. We thus developed and implemented a science learning environment for children in the first years of schooling which contains structured learning materials with the goal of supporting conceptual change concerning the understanding of the floating and sinking of objects and fostering students' scientific reasoning skills. In the present implementation study, we aim to provide a best-practice example of early science learning. The study was conducted with a sample of 15 classes of the first years of schooling and a total of 244 children. Tests were constructed to measure children's conceptual understanding before and after the implementation. Our results reveal a decrease in children's misconceptions from pretest to posttest. After the curriculum, the children were able to produce significantly more correct predictions about the sinking or floating of objects than before the curriculum and also relative to a control group. Moreover, due to the intervention, the explanations given for their predictions implied a more elaborated concept of material kinds. All in all, a well-structured curriculum promoting comparison and scientific reasoning by means of inquiry learning was shown to support children's conceptual change.