We play tested this with 10 year olds at the San Francisco Public Library recently and the kids LOVED it -- keep in mind, these were not middle or upper middle class privileged kids, but lower income at risk ones.

The challenge is that after about 45 min of "play" time, they want to 'build' something and create--and Playgrounds doesn't provide the environment to do that. Then, they get bored and feel encumbered.

After working with over 1000 kids in our beta phase, we've tested almost every product on the market.

I personally like Scratch + Learntomod.com which uses Minecraft to teach Javascript and CodeCombat.com (My YC batchmates).

We're soon launching an online course (for a nominal fee) that has progression, milestones, and support for parents who want their kids to 'complete' a project. Message me or check out our beta site if you're interested to in being a beta tester for our online product--would love to hear from you.