The awareness for preserving privacy in in-home monitoring robots is increasing. Although several studies have proposed privacy-preserved in-home monitoring robot systems for adults, only a limited amount of attention has been paid attention to research on privacy-preserved in-home monitoring of babies. Like previous studies, thermal infrared image-based methods could ensure a privacy-preserved monitoring of babies, yet when existing detection methods were applied to thermal images to detect babies and adults, we discovered a frequent occurrence of misdetection due to the presence of thermal residual heat marks. In this research, we propose a sequential thermal image-based detection that conjugated the characteristics of thermal residual heat marks. The proposed detection reduced misdetection caused by thermal residual heat marks by 98.7% when compared to RetinaNet. In addition, we open-source our collected thermal image-based baby and adult dataset via: https://github.com/donkeymouse/ThermalAdultandBaby.