I want to know, how do you keep them from touching the microwave now?
It seems to me that you think granting access to the microwave will create an option to abuse that privilege - which they don't now have, since they now can't touch the microwave at all. I would expect some trial and error, but I would, like I said above, constrain experimental usage by expecting payment. Along with not getting cooking lessons, I did not get an allowance, so replacing a microwave might still be on my list of things to pay my parents back today. If I were a parent, I would supervise and expect a certain amount of trial and error (within the recipes they are allowed to cook), and I would have a steep penalty for deviating from constructive usage of the microwave, especially (but not only) if it resulted in destruction of the machine itself. Not just replacing the machine, but the alternative non-cooked/warmed snacks he would have to eat if it were rendered disabled - and you have no responsibility to make sure these are palatable.
In my non-kid-having mind, I say "fuck. You're eating carrots. Not even those baby carrots, but peeling your own full adult carrots." I suppose a kid-having mind might substitute a string cheese. It's not up to me. Maybe I would see how they handle a peeler before they handle a microwave oven, since they can't break the peeler.