 For all children, visits are scheduled at regular time points see Table 1. Enrolled children who are not treated or those with premature discontinuation of rivaroxaban will at least be seen at the end of the study treatment period. During all contacts, the treatment and clinical course of the child will be evaluated. Children with suspected efficacy or safety outcomes will undergo confirmatory testing as per standard of care. Blood samples for PKPD will be taken at defined time points see Table 2. 