The 6 sons born to Devaki and Vasudeva were actually 6 sons of Prajapati Manu in the millennium of Swayambhur Manu. They were killed one by one just after their birth by their uncle Kansa, brother of Devaki, who was destined to die in the hands of the 8th son of Devaki & Vasudeva.
After getting killed by Kansa, their soul went to a lower planetory platform Sutala where demon king Bali(Mahabali), son of Virochana, grandson of Prahlad, great grandson of Hiranyakashipu, was appointed and sent as a ruler by Vishnu's Vaaman avtar when Bali gave away everything under his rule to Lord Vaaman in charity.
We have a reference of this in Bhagwat puran. One day, after Krishna and Balaram returned from Kurukshetra to Dwarka, they went to visit their parents in their palace. Vasudev and Devaki were very glad seeing their sons after a long time. They knew that their sons were no ordinary human beings. Vasudev remembered one incident which he heard a long time ago that Krishna and Balaram had brought back the dead son of their Guru Sandipan muni. Since Devaki had also heard this incident, she had been also thinking of her own sons who were killed by Kansa, and while remembering them she became overwhelmed with grief. She then asked Krishna to bring back his other sons from death just as he brought his teacher son. After hearing Their mother request Lord Balarama and Krsna immediately called for the assistance of Yogamaya and started for the lower planetary system Sutala which is ruled by Bali raja. Bali with his whole family welcomed Krishna and Balaram and worshipped them as Vishnu. After receiving their prayers, Lord Krishna spoke as follows :
"My dear King of the demons, in the millennium of the Svayambhuva Manu, the Prajapati known as Marichi 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(in Sutala). 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 Krishna and Lord Balaram took away the six souls of their elder brother to Dwarka and presented them as small babies to their mother. 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.
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 Devaki, which had touched the body of Lord Krishna, 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.
