Peanut Butter and Jelly Ending
I think the "peanut butter jelly sandwich challenge" is a great lesson in general, but it's especially great for computer science. Kudos.

That being said, I think you might be conflating two separate topics that are each worth their own lessons:


The exactness of syntax: take rightHand plate and take plate rightHand are two different things, and a computer might not be able to understand what you mean.
The exactness of semantics: take plate rightHand might sound like an obvious instruction to a human, but that's because we've had years of practice picking up plates with our hands. But a computer hasn't had that practice! What if it tries to pick up the plate with a fist? What if it tries to grab a plate by the middle instead of by the edge? So often our instructions to a computer have to be much more detailed than we first thought.


The "semantic exactness" part is the real beauty of the peanut butter jelly sandwich challenge, imho. One of the hardest things to do in computer science is take things that humans "just do" and break them down into individual steps. If they continue exploring CS, your students will eventually have to ask questions like "how would I sort a stack of index cards with numbers written on them?", and their initial reaction will be "I don't know, I just do it!". The peanut butter jelly sandwich challenge teaches them to dig a little deeper than that first reaction, and to start breaking things down into smaller steps.

And the fun part is that no matter how exact students make their instructions, you can always find a way to get it wrong.

Anyway, back to your actual question: I would propose a three-part lesson:


Step one: Have students come up with "programs" that they enter into your engine. This might be a homework assignment. This teaches them the "syntactic exactness" part, and gets them thinking about breaking things down into smaller steps.
Step two: In class, have students volunteer to read off their programs. You follow their instructions, pointing out as many inconsistensies as you want, in as messy a way as you want. After a few examples, take a step back and give them the lesson that even if your syntax is valid, sometimes your semantics can be unclear. Then maybe have them abandon your syntax and give you instructions in English, that way they can be more specific about each step. But like I said before, no matter how exact their instructions are, you can still find a way to get it wrong.
Step three: After a few rounds of this, have students break off into pairs: one student giving instructions, one following them and getting it wrong in the same way you did. Whether you give them real ingredients or tell them to pretend they have real ingredients is up to you and how much of a mess you want to clean up afterwards.