The mathematics curricula of countries such as the United States, Singapore, Australia and New Zealand have started to incorporate the teaching of algebra in early childhood education. From this perspective, our study has two objectives: (a) to design a learning trajectory and teaching itinerary introducing patterns for children between the ages of three and four years old; and (b) to apply the previously validated learning trajectory and teaching itinerary to a group of 24 children between three and four years old from Spain. The results obtained through a design-based research methodology indicate that children recognize and represent seriations that follow a simple pattern in different teaching contexts. It is concluded that the trajectory and itinerary empower a specialized educational intervention to promote the initiation of algebraic thinking in early childhood education.