ChildRepository.cs

public class ChildRepository {
  public Optional<Child> GetOptional(string identifier) {
    //Very simple parent-child example
    
    var childStream =_streamReader.Read(identifier);
    var parentId = ((SomeInitialEvent)stream.Events[0]).ParentId;
    var parentStream = _streamReader.Read(parentId);
    
    var root = new Child();
    root.Initialize(parentStream.Events);
    root.Initialize(childStream.Events);
    return new Optional<Child>(root);
    
    //there's no limit to how one can weave together various streams
    //and use that union to build up the state of a particular aggregate
    
    //not only can one vary along the 'multiple classes per stream' axis
    //but also along the 'multiple streams per class' axis
  }
}

KISS! I like it!

This is great, if you don't mind some of the infrastructure, leaking into the domain layer ...