Based on a new concept of Kinder-GuaRdian which aims to realize a support system for kindergarten's staffs and parents by watching over children's activity and their growing process, activity recognition (AR) for children is treated in this paper. In case of children, smaller amount of sensors is better since easy fitting of the sensor device to child's body is strongly preferable. In this study, using one acceleration sensor attached to the upper arm, AR in cases of adults and children were evaluated using several orthodox classifiers and self-organizing map (SOM). Through the several types of validation, effectiveness, issues, and benefits of their methods are discussed.