Young children who are called upon to donate regenerative tissue – most commonly bone marrow – to save the life of a sick relative are in a unique position. The harvest of tissue from them is non-therapeutic and carries the risk of physical and psychological harm. However, paediatric donation is relatively common medical practice around the world. Where some doubt exists over the legality of allowing a child to donate, courts can be asked to authorize the procedure and in doing so will apply the ‘best interests’ test in making their decision. How are a young child’s rights recognized in such a situation? This article considers whether the best interests test is the ‘best’ test to be applied by courts when cases of potential child donors come before it. The approach of courts in three jurisdictions is analysed, and problems in the application of the test in this context are discussed. While the continued use of the test by courts is supported, the way the test has been used by courts is critiqued and recommendations made to better respect the rights of the potential donor child.