There are two primary goals of this multicenter clinical trial that combines an FDA device trial and a phase II drug trial in the same study cohort. These two goals are to: 1. To evaluate the safety and effectiveness of the Cardiohelp Device for VA-ECMO (heart-lung support) for up to 30 days of support in children with severe heart failure with the goal to support its FDA clearance in children. 2. To evaluate heparin versus bivalirudin as the primary blood thinner (anticoagulant) in a randomized trial of children supported with the Cardiohelp ECMO System with the goal to plan a phase III (pivotal) randomized clinical trial The main questions the Cardiohelp single-arm trial seeks to answer are: * What is the safety and effectiveness of the Cardiohelp device for pediatric ECMO? * Should the Cardiohelp device be FDA-cleared for children based on the results of the study? * What are the optimal performance specifications of the Cardiohelp device in children? The main questions the blood thinner randomized trial seeks to answer are: * Which blood thinner is more promising (i.e., more effective and safer) in children on the Cardiohelp device? * How should a pivotal trial of heparin vs. bivalirudin be designed so it is the most informative and efficient to determine the best blood thinner? Children who are receiving the Cardiohelp device will be approached and consented to participate if interested. For the Cardiohelp device trial, participants will undergo a standardized data collection to estimate survival to 30 days and the prevalence of serious adverse events like stroke, bleeding, and hemolysis. For the blood thinner randomized trial, participants will be randomized 1:1 to blood thinner strategy to determine which blood thinner has the fewest bleeding and clotting complications. For the Cardiohelp single-arm trial, participant outcomes will be compared to performance goals (PG) derived from the ECMO literature. For the blood thinner randomized trial, the amount of bleeding and clotting will be measured. The study is funded by an R01 grant from the FDA's Office of Orphan Product Development (OOPD).