You can use EXISTS
:
SELECT ChildDummyTable.Id ChildDummyTableId,
ParentDummyTable.Id ParentDummyTableId
FROM [DummyTable] ChildDummyTable
INNER JOIN [DummyTable] ParentDummyTable
ON ParentDummyTable.Id = ChildDummyTable.ParentDummyRowId
WHERE EXISTS(SELECT DummyRow2 FROM DummyTable2
WHERE DummyTableId=ChildDummyTable.Id
EXCEPT
SELECT DummyRow2 FROM DummyTable2 ParentDummyTable2
WHERE ParentDummyTable2.DummyTableId=DummyTable.Id)