Is it possible to retrieve list of subprojects using PSI (project server interface)? I found Relations datatable in ProjectDataSet but it is always empty. Maybe there is an easier way to get guids of subprojects than using PSI?
MSDN says that Relations table is used to map tables in dataset, not projects. http://msdn.microsoft.com/en-us/library/websvcproject.projectdataset.relations_di_pj14mref.aspx. Try to check Task data table. It will contain summary tasks for each sub project. Maybe it also contains sub project id.