To augment the prior answer, anything of this nature requires some JavaScript in the browser to do it. Your model design alone isn’t enough.
In the general sense, some JavaScript (direct or using the framework of your choice) needs to be triggered by the on-change event of the first drop-down. That JavaScript would issue an AXAJ GET to a custom view in Django to retrieve the next widget’s data (or an HTML fragment for the options). The view returns the data back to the JavaScript, which then injects the data into the page.
There are some third-party packages that can be plugged into your application, but you probably want to find one that matches whatever JavaScript framework you’re currently using.
Also see: Django: how to choose from a list based on the previous selection? for one such implementation.