There are many approaches to assist students in developing their research topic; you might help students expand on what they already know or direct them to resources to get ideas and background information on topics others have looked into. Talk with the professor/TA about what approach they prefer.
Topic development is necessary to teach when:
Here are a few free online concept map tools that you could have your students use during the session. These tools do not require an account unless you want to save the map.