Scratch allows children to learn computational thinking concepts through a creative computing experience. Keeping it simple and focusing on the creative aspect of programming is a major emphasis of Mr. Resnick's team at the Lifelong Kindergarten Group.
The kids that choose to continue with programming eventually reach a wall with Scratch, where the block-based aspect breaks down. At which point they are ready for syntax-based languages. However, it's amazing how complex some of the featured projects are on the Scratch website with regards to both art and logic.