Hi David Sanchez,
By default, in the MasterDetailsViewDataGrid, each child contains a different DetailsViewDataGrid. Please refer to the below screenshot.

According to the implementation, SfDataGrid SelectedItems do not persist when the record is selected in another DetailsViewDataGrid or ParentGrid. We regret to inform you that we do not currently support keeping SelectedItems while selecting multiple rows in different DetailsViewDataGrid or ParentGrid.
Regards,
Dhanasekar M.