I guess it's too late for any input, but just in case:
Ask them to compare django to other frameworks, likes, dislikes
Ask them about a few packages they used before
Have they written their own migrations or package
How would they deal with a slowly loading page?
Do they know about SQL? On what level? Have they used advanced features like views/materialized views, triggers, lateral joins, Subqueries. Can they tell you about different kinds of joins.
Figure out if they know the basics of HTTP and forms
Ask them about html and css stuff
How would they go about implementing a telephone book application? (or any other pretty simple application). Have them sketch a rough plan on what to implement and test when. eg. Make models first, then a view and a template, etc.
Have them explain what CBVs can help you with.
I know, it's vague. You want to get a good picture about how deep their understanding of the web in general is and also see if they have used django beyond very simple tutorial projects.