After Lord Krsna heard the prayers of Bali Maharaja, He spoke as follows: "My dear King of the demons, in the millennium of the Svayambhuva Manu, the Prajapati known as Marici begot six sons, all demigods, in the womb of his wife, Urna. Once upon a time, Lord Brahma became captivated by the beauty of his daughter and was following her, impelled by sex desire. At that time, these six demigods looked at the action of Lord Brahma with abhorrence. This criticism of Brahma's action by the demigods constituted a great offense on their part, and for this reason they were condemned to take birth as the sons of the demon Hiranyakasipu. These sons of Hiranyakasipu were thereafter put in the womb of mother Devaki, and as soon as they took their birth, Kamsa killed them one after another. My dear King of the demons, mother Devaki is very anxious to see these six dead sons again, and she is very much aggrieved on account of their early death at the hand of Kamsa. I know that all of them are living with you. I have decided to take them with Me in order to pacify My mother Devaki. After seeing My mother, all these six conditioned souls will be liberated, and thus in great pleasure they will be transferred to their original planet. The names of these six conditioned souls are as follows: Smara, Udgitha, Parisvanga, Patanga, Ksudrabhrt and Ghrni. They will be again reinstated in their former position as demigods."
And thereafter Lord Krsna and Lord Balarama took away the six conditioned souls and returned to the city of Dvaraka, where He presented them as little babies before His mother, Devaki. Mother Devaki became overwhelmed with joy and was so ecstatic in motherly feeling that immediately milk began to flow from her breasts, and she fed the babies with great satisfaction. She began to take them on her lap again and again, smelling their heads and thinking, "He has gotten my lost children back!" For the time being she became overpowered by the energy of Visnu, and in great motherly affection she began to enjoy the company of her lost children.
The milk from the breasts of Devaki was transcendental nectar because the same milk had been sucked by Lord Krsna. As such, the babies who sucked the breasts of Devakiji, which had touched the body of Lord Krsna, immediately became self-realized persons. The babies therefore began to offer their obeisances unto Lord Krsna, Balarama, their father Vasudeva, and mother Devaki. After this, they were immediately transferred to their respective heavenly planets.
After they departed, Devaki became stunned with wonder that her dead children had come back and had again been transferred to their respective planets (source: Bhagwatam 10.85)