This paper describes construction of indoor location service for searching lost child and preventing lost child. Conventional indoor location services have only considered searching lost child or preventing lost child. Therefore, we construct indoor location based service of searching lost child and preventing lost child. Specifically, we develop a Kid’s friendly design wearable device which have radio communication device and many sensors. We construct indoor location service in terms of estimating position by using the device. In using this indoor location based service, a parent always locate where parent’s child over the Web. Moreover, parent’s mobile phone is called when child is constant length away from parent. In this paper, we show the localization system by using radio communication device and probability model, and show the experimental results in shopping mall.