Abstract Age assurance is a way to prevent children accessing content, products or services that are potentially harmful to them, ranging from gambling services or alcohol or tobacco or, increasingly, certain products and services online. Now that children’s lives are mediated by digital technologies, policymakers are deliberating over the legal, technical and practical challenges. These have been little examined from the perspective of children’s rights. By combining legal and social research methods, this article examines the legal requirements for age assurance in Europe, assesses compliance by companies and reveals the consequences for family life. In law and practice, we show that age assurance is often ineffective in protecting children from online risk of harm. Further, as currently implemented it risks children’s other rights – to non-discrimination, privacy, to be heard, and their civil rights and freedoms, and remedy. We identify promising directions for the use of age assurance in child online protection, focusing on European policy, regulators and civil society actors.