At this time, it´s not possible to use variants or hirachical data at a child table, because both use the column pid...
May be we can change the builder https://github.com/MetaModels/core/blob/master/src/MetaModels/DcGeneral/Dca/Builder/Builder.php#L656 to use self definited column(s) as parent id
may be this old row https://github.com/MetaModels/core/blob/84b229f46ddbfcc112278a228c893cf63ae6d791/src/MetaModels/DcGeneral/Dca/Builder/Builder.php#L656
At MM 2.3 it´s possible to use variants as data at a child table