Trying to find a good word for 'a small discussion' or perhaps 'a focused discussion'

Depending on the context, this could be called a breakout session. This term is typically used at academic/industry conferences, and refers to time set aside for smaller groups of people to discuss a narrow focus area related to the main topic of the conference. It's useful for conveying the focused nature of the discussion compared to the relatively broad content of the meeting.

You want to check in with your colleagues [on _______ (topic)]. That implies a specific, focused discussion, in which each person speaks only briefly.

I couldn't find the right definition, I'm sorry.

Example: "Let's check in again in a week's time, to see how everyone is doing with their assignments."

May I suggest the word confab as in the following scenario:

The team from the PR department did a great job yesterday, holding a confab over lunch that produced the outline of our new advertising campaign.

confab: informal "An informal private conversation or discussion";

North American "A meeting or conference of members of a particular group."
(Oxford Living Dictionary)