UPDATE ChildObject
SET Attribute1='<process_name>'
WHERE ObjectId = ANY (
    SELECT TOP 100 ObjectId
    FROM ChildObject alias1
    WHERE State = 0 AND NOT EXISTS (
        SELECT *
        FROM ChildObject alias2
        WHERE alias2.Attribute1 <> ' '
          AND alias2.Attribute1 <> '<process_name>'
          AND alias2.ParentId = alias1.ParentId
    )
)