The SuperTux game engine has seen a near-complete rewrite that now uses OpenGL, OpenAL, SDL2, and other modern technologies. The SuperTux 0.4.0 game is now able to handle translations, an in-game manager for downloading add-omns, new maps / in-game content, improved sound effects, new graphics effects, many new game objects, and a range of other improvements.
For those that don't know, SuperTux is inspired by Nintendo's Super Mario but features Tux rather than Mario. Likewise, if looking for another open-source game this holiday for kids, there is also SuperTuxKart if racing is more your thing.
More details on SuperTux 0.4 can be found via the project's GitHub.