Just to make sure, I magine you have already, but make sure to read the docs in FlexCel.NET of VirtualDataTable and VirtualDataTableState (the 2 classes you have to implement). They are in appendix II here:
http://www.tmssoftware.biz/flexcel/docs/net/UsingFlexCelReports.pdf
And while for .NET, they are basically the same.
The complexity comes from what you want to support: Stuff like allowing Lookups in the data or Distinct need to implement corresponding methods, but if you don't want, you don't need to implement them. The specific functionality just won't be supported in the report. (for example the user won't be able to use DISTINCT in the config sheet if you don't implement the corresponding method.
To implement implicit child lists you need to override GetRelationWith and return the related tables there.
I'll try to come up with an example for the weekend.