This is a great unit if you are bringing attention to the upcoming presidential election. It gives grade schoolers the opportunity to learn debating skills revolving around an election that they are undoubtedly aware of.
I suggest you choose the groups for each candidate (or do so randomly). I would suggest groups of 4 to 7 students. In my class, I had two Democrat groups and two Republican groups. One Republican group debated one Democrat group. So, in essence, there were two debates in my room.