Aiming at the increasing security risks of children, this paper presents and implements a kind of Mobile Children Security Monitoring (MCSM) system based on android phones to help guardian to acquire whether children are safe or not. MCSM implements the software hand function and the danger zone function for two typical safety scenarios, i.e., going outside with their guardians and without their guardians respectively. The software hand function can keep children in guardian's view by using Bluetooth near field communication, and the safety zone function can make guardians know children's location timely by using GPS sensors, acceleration sensors, and mobile GIS (Geographic Information System). Experiments shows the system has the characteristics of high reliability, short response time and high accuracy, and can meet the requirements to ensure children's safety.