Regarding the Main :: Sub v. Parent :: Child discussion. I'm thinking that "Main" could easily be construed as the Single (top of the tree) Main Event. In that case, the event below (Day) is a sub-Event. Does that make the partial day Event a sub-sub-Event? People better understand that the Parent :: Child :: relationship is generational. Since we have more than one generation (and possibly more to come), I'm thinking Parent :: Child is optimal.
How would we have more - you ask. In next update we might have a morning Child event that has two, or more, different Speakers with two separate sign-in/outs.
So, unless you see a reason not to, let's go with Parent :: Child.