This work aims to explore the application of the physical and digital twin pair as a computational thinking instrument to support learning and exercise proportional reasoning at the ages at which pursuing STEM studies is decided. Diverse studies reveal that the initial decision about the vocation in studies is made towards the end of the primary education stage, also from 8-9 years old. The Digital Transformation Commission of the Engineering Associations of Catalonia is concerned about the low number of young people who decide to pursue STEM studies. STEM explorations in early childhood lay the foundation for lifelong learning. In addition, from age 5, children informally begin to practice proportional reasoning, and in the third year of primary school (8-9 years old), the subject is formally introduced into the Primary Education curriculum. The notion of proportionality is very transversal, and it is one of the fundamental concepts of the STEM field. The consequences of an incomplete understanding of it directly or indirectly affect the attitude of subjects toward mathematics and the STEM world. With the emergence of Industry 4.0, the digital twin concept is becoming one of the central instruments of Digital Transformation. In this paper, we propose using computational thinking to help the understanding of proportional thinking using a physical and digital twin pair. In 2022, computational thinking was introduced into the Spanish and Catalan educational system's early childhood, primary, and secondary education curricula. It includes programming with block languages such as Scratch or Snap!. A digital twin of the system has also been implemented in Snap!. Through the collaboration of a primary school, a workshop has been prepared in which they exercise the concepts of proportional reasoning, such as ratio, proportion, scale, or percentage. The approach followed has consisted of developing a physical system of a floating ball, controlling the electric motor's power with an electronic system based on low-cost elements from the maker world. We have designed a very easy-to-use system for any teacher, and its open-source nature ensures its transparency and documentation. The foam ball rises and remains floating at a height determined by the power supplied to the motor. The control of the system is carried out with Snap!. It will be assumed that the engine revolutions are proportional to the power setpoint. The physical system generates a high level of expectation and attention in the students, facilitating the process of its abstraction. The range of options of the developed system allows experimenting with the actual system and a schematic digital twin with sound effects. In all cases, the same computational thinking tools have been used to exercise the concepts and powerful ideas of proportional reasoning. At the end of the workshop, the understanding of the concepts exercised and the assessment of their personal experience were evaluated. This gave us helpful information to motivate students of these ages and below to maintain their STEM vocation.