This may be a lost cause, but I don't think a beginner language should have mutable data at all. The only reason we really need mutability is for speed - our programs have to run on actual machines and for a long time we didn't have efficient ways of working with immutable data structures. It's a lot easier to reason about immutable data. And any non-programmer is not going to expect mutable data - from the beginning when they started learning math all the way up to MS Excel wizards, all the data has been immutable.
Mutation is a deep magic that learners shouldn't start with. It's pure premature optimization. Also everything should be expressions, no statements, why force people to learn extra syntax? Thank you for listening to my TED Talk.
...but thank you for making this tool! I love the IDE - simple, clear, and self-documenting!