I wanted to write about Pink Pony for a long time, but never came around to it. It's a Tron-like racing game and features hearts, ponies (also ai-controlled ones), graphic shaders and split-screen multiplayer support. Your aim is to collect hearts and to not run into your foes' trails. Last pony standing wins!
Download it for Win32 or as an easy-to-compile source tarball.
Unfortunately, the game does not support older graphics cards. If you are capable and willing to add legacy support, you can find the source repository here. If you have ideas for more features, please post them (or just comment this blog article).
Pink Pony development was not a one-man ride: While the game concept and code is by ginkgo, tornado created the pony model and textured it, then Skorpio added animation to the pony. A sky render by brisingre is used in the game and I contributed two textures to it. Edit: I forgot to add that MCMic programmed the AI and that Renich's music is used.
I believe that Pink Pony now is the best child-friendly open source game, regarding visuals and gameplay. Most free software games friendly for kids I know try to be education software. Then again, maybe I should give Tux Racer another try.