DOI: 10.1145/950566.950589This paper presents an overview of the lessons learned building Disney's Toontown Online, a 3D massively multiplayer online game (MMP) for children ages seven and older. The paper is divided into three main parts. The first presents design highlights of Toontown Online and focuses on the challenge of building an MMP for kids. In particular, we discuss ways of incorporating kid-friendly socialization into an MMP. The second part of the paper presents an overview of Panda-3D, the VR Studio's open source 3D graphics engine. We focus on the aspects of Panda-3D that helped to facilitate the development of Toontown. In particular, Panda's expressive, platform-agnostic scene graph architecture, and flexible scripting language tools for distributed storytelling. We finish with an overview of Toontown's server architecture and present our nothing-but-net strategy for downloading a full-featured 3D online game.