Is this a suitable case in which using dynamic function creation would be justified?

I’m currently developing a tutorial site for teaching the fundamentals of Web development (HTML, CSS, and JavaScript, for starters). I’d like a setup where I could give in-depth coverage of all sorts of topics and then provide a basic sandbox environment where the user could write code which solves the question asked at the end of each tutorial section.

For example, if I’d covered multiplication in a previous tutorial, and the user had just finished a lesson on functions being capable of returning values, I might request that they submit a function which returns the product of two parameters.

Is this not the perfect instance in which using dynamic function creation would be considered a good idea? Let’s look at an example.

function check()
eval('var f = ' + document.getElementById('user_code').value);
if (f(5, 10) == 50)
// user properly wrote a function which
// returned the product of its parameters

Is this at all a bad idea? If so, please explain.