Display field collection items together with nodes in view sorted by date
I would strongly suggest to move away from field collection implementation. Because, Sub - Events are actually events and should be nodes. Right now they are becoming values of a Single Event by using field collection.

Try considering Entity Reference. Add a reference field called Parent Event to the event content type and make it reference to same content type. Make the field optional so that you can create root Events.

In future, above architecture will provide you lot of advantages. Hope you like it!