Coding (or computer programming) helps equip children with an intellectual structure that is valuable for their lifelong learning and development. The proliferation of innovative coding platforms, especially screen-free programmable robotics, has made it possible for coding to be integrated into early childhood education (ECE). However, how the coding curriculum has been designed and used in ECE settings, as well as its effectiveness, is understudied. This scoping review evaluates, synthesizes, and displays 20 studies on coding curriculum in early childhood published in 2012--2021, involving curriculum design, coding platforms, pedagogical approaches, research methods, and research findings. The review contributes to a mapping of existing work focusing on coding curricula in early childhood, thus demystifying and clarifying the characteristics and effectiveness of these intervention programs. Its findings also shed light on the improvement mechanism and solutions of early computing education in both quantity and quality.