IEventParentRow interface is a base class representing any row in the event sheet that can have other events nested beneath it. For example an event group is a parent row since it can have other events nested inside it, but an event comment is not a parent row, because nothing can be nested inside it. Note that the root node of the event sheet is a parent row.
Add an empty child event block, with no conditions or actions. Returns a promise resolving with the created IEventBlock.