I would say that in general I try to put as much data into a single dataset as possible to minimize subreports or other even more complex ways of presenting one-to-many data. A single master tablix with grouping levels (sometimes parent/child and sometimes adjacent grouping levels) is my standard approach to any such report design.

I am glad that my suggestion got you past a difficult barrier in design. Nice to know that I could contribute in part to your ultimate success.