Scratch is awesome but the current version requires Flash to play. Here are my family's profiles:
Some day Scratch 3 will be out and it will support HTML5 instead of Flash.
There's also ScratchJr, which is a tablet app for children as young as five.
Hopscotch is very similar to Scratch but it's iPad only and you can't publish your work to the web.
Scratch is open source. Version 2 can be found at https://github.com/LLK/scratch-flash but I'm holding off until Scratch 3 which seems to be at mostly at https://github.com/llk/scratch-blocks according to https://scratch.mit.edu/developers . The ScratchJr source code can be found at https://github.com/LLK/scratchjr
If your kids are ready to advance beyond Scratch, I recommend Phaser.