Internet of Things (IoT) devices are becoming ubiquitous and gradually impacting on young children's play, learning and growth worldwide. It is vital for educators and parents to understand how the IoT devices have been used and with what consequences. Attempts have been made by researchers to explore varied IoT device usage in ECE, but there lacks a consolidated review on this topic. Therefore, we conducted a systematic review on the IoT device deployment in ECE using four major databases over the past 20 years. A qualitative synthesis was performed to analyse the data extracted. The results revealed that for young children from birth to 8 years of age, the IoT devices were normally used as IoT playthings. Additionally, the IoT devices could provide the young children with opportunities to connect digital and physical worlds for their playful explorations, help them to build their knowledge base, arouse their interest and enthusiasm, and encourage them to be autonomous learners. No negative influence associated with the children's IoT device use was identified in the selected articles. However, high prices and data security were two concerns raised as influencing the educators' and parents' IoT device adoption. The findings may serve as a rationale for practitioners' and parents' decision making. Implications for future research are also suggested.