As we know the 8th Child of Devki or Shri Krishna himself was the God himself, but does Shri Krishna realise what happened to his slain brothers or sisters? Are there any special mentions apart from them being killed at the hands of Kamsa?
The embryo was moved from Devaki's womb into the womb of Rohini and thus 7th child Balarama was born from Rohini.
The six children's details are mentioned in Śrīmad-Bhāgavatam (Bhāgavata Purāṇa) » Skandha 10: CHAPTER EIGHTY-FIVE:
Text 47: The Supreme Lord said: During the age of the first Manu, the sage Marīci had six sons by his wife Ūrnā. They were all exalted demigods, but once they laughed at Lord Brahmā when they saw him preparing to have sex with his own daughter.
Texts 48-49: Because of that improper act, they immediately entered a demoniac form of life, and thus they took birth as sons of Hiraṇyakaśipu. The goddess Yoga-māyā then took them away from Hiraṇyakaśipu, and they were born again from Devakī’s womb. After this, O King, Kaṁsa murdered them. Devakī still laments for them, thinking of them as her sons. These same sons of Marīci are now living here with you.
Text 50: We wish to take them from this place to dispel their mother’s sorrow. Then, released from their curse and free from all suffering, they will return to their home in heaven.
Text 51: By My grace these six — Smara, Udgītha, Pariṣvaṅga, Pataṅga, Kṣudrabhṛt and Ghṛṇī — will return to the abode of pure saints.