Are we attached to Thing's property name "SubThings" or can we rename it to "Children"?  The meaning might be more obvious to newcomers.

Ok, I'll change it in a few minutes.

EDIT: Done.  However, the PlayerDocument equivalent is still named "SubThings" because renaming it broke player loading, so that particular rename should occur in conjunction with a RavenDB database update which I'm not sure how to do offhand ATM.  Fortunately that can wait, and I just slapped a TODO on that property.

After that, how do you feel about the ...EventArgs being renamed to ...Event? Our events are more than structures so calling them 'args' classes is not only a bit redundant, it's also a bit misleading.

GameEventArgs becomes GameEvent, CancellableGameEventArgs becomes CancellableGameEvent, RemoveChildEventArgs becomes RemoveChildEvent, etc.

Sounds reasonable. Make it so.