First of all, before even clicking through, my reaction to the name was negative. To quote henrikberggren's comment: "Naming a learning platform 'Sugar' is like naming a rehab clinic 'Heroin'."
Second, those stick figures look like skull-and-crossbones.
Third, your sample "fun activity" looks less fun than anything a kid with a computer has access to.
Fourth, creating a new user in your Sugarizer demo is an awful experience. It asked me to pick a name first, then let me go through the rest of the steps -- including "choosing" a new avatar color by clicking to get random colors until I just gave up -- and then, at the end, told me the user "foo" already exists.
Fifth, I don't find the UI intuitive at all. When I'm inside an activity, if I want to go back to the activity selection page, I have to click on an octagon with a square inside it. No idea how I'm supposed to associate that with "home" or "back" or any of the verbs I could think of.
Lastly, I tried out a few of the activities and I have no idea how any of that is supposed to help kids learn or collaborate. The "clock" activity just displays a clock. That's it. The "tam tam" activity has a confusing UI, but once you overcome the confusion, you can just pick different instruments to hear their sound. The most confusing was the "maze" activity, because I pressed one arrow key and it solved the whole maze. Later I realized that it follows the path until a decision point and then waits for input, but it's really weird.
I appreciate that this must have taken substantial effort, but I have no idea just how this is supposed to be useful. And let's face it, I gave it more time than an average visitor probably would.