The main purpose of the Kids-DOTT trial is to provide key evidence for the optimal duration of anticoagulant therapy for thrombosis in children, given that the conventional duration of such therapy in children is derived solely from evidence in adult thrombosis trials. Study hypothesis: Among children with first-episode acute venous thrombosis in whom thrombus resolution is evident following the initial 6 week period of anticoagulant therapy, the cumulative incidences of recurrent venous thromboembolism (VTE) and post-thrombotic syndrome (PTS) at 2 years is not significantly different between those receiving 6 weeks and those receiving 3 months total duration of anticoagulant therapy.