This study is for young children with severe hemophilia A who have previously not been treated with BAX855 or other FVIII concentrates. The main aim of the study is to check for side effects from treatment with BAX855. This includes the buildup of antibodies against FVIII which may stop BAX855 from working properly. Another aim is to learn how well BAX855 controls bleeding. In this study, the children can receive BAX855 either as preventative treatment (prophylaxis), or as needed to treat bleeding (on-demand). In case a participant develops antibodies, treatment will be provided as part of the study.