Blockly is a graphical programming opensource (Apache 2.0 License) tool developed and supported by Google. The interface is very close to Scratch that many children common with.
Blockly is completely server independent and can be easily customised. From our side, we developed components that connect to the robot and set of blocks for robots motors, camera and sensors. Children can code their robot in a Scratch way using these visual blocks and build a complex robot behaviour they want.