@ Janaki - According to SBP,Vasudev & Devaki were House Arrested.Kans made their house a Prison.
1) 'Kamsa understanding the essence of what he said was for the time being stopped from killing his sister. With him more at ease, Vasudeva then was happy to come home [unharmed].
2) Kamsa thought that all the Yadus were divine and that therefore any child born from Devak could be Vishnu. He thus in fear of his own death arrested Vasudeva and Devak, confined them at home in shackles and killed one after the other each of their newborn sons not knowing whether it would be the 'Never-born' Lord or not.
3) Devak with in her womb the Sustainer of All the Universes, could, being confined in the Bhoja house like the flames of a covered fire or like the knowledge of a man unable to express himself, not freely radiate her light.
Krishna's birth was a result of mind contact.
He, the Blessing of the Complete Universe who is Infallible in All His Parts, was thereafter by the son of S'rasena [Vasudeva] in full transferred from mind to mind to his dev [Devak] so that she, preserving the Supreme Soul and Cause of all Causes, bloomed of happiness like the eastern sky.
Perhaps Balram was born in the same way.
@Sruja - You're Right who can else do all this except Hari,who runs this universe with his Maya Shakti and we are mere Puppets in his hand and yeah Yogamaya was said to have played a great role for Vishnu to incarnate as Krishna and managed all the related incidents.As you said it is difficult to predict or explain when,where,how&why hence Sage Vyasa termed them as Hari's Illusion Power.
@Lola - I loved reading your explaination!
Actually I was trying hard to understand these incidents.Vasudev grew strong mentally coz of the circumstances around is what I believe.
According to SBP,in the beginning Vasudev thought this way...'If I promise to deliver my sons to this man of doom, I might set my innocent Devak free. Perhaps I don't get any sons or maybe he[Kamsa] will die beforehand. That might happen or the contrary. Who knows what fate has in store for us? That is difficult to say. Even though the threat remains hereafter, I, at least for the moment, may avert her death.'
Here the Divine prophecy declared that Devaki's 8th Child would kill Kans.Words of divine will never be false.Though he made a promise at that moment to save Devaki,Vasudev was unaware of this truth(Divine will).He was still hoping, 'what if the contrary happened!?'
When Kans gave back his first child saying,'You can take this child back, my fear does not concern him, my death was predicted from the eighth pregnancy you have with your wife' Vasudev left without attaching too much value to the words of that untruthful character [Kans]lacking in self-control.We find an evolution in his character,i.e;trust not a fool.
Perhaps,the first 6 children were out coz of ordinary human emotions,physical relations,but BalaRam & Krishna were of higher origin!And this level was acheived by Vasudev & Devaki with certain spiritual developments within them.They were moulded gradually to meet the requirements of producing a Divine Avatar!
Now I understand that because of this spiritual growth Vasudev realized that a Father's Dharma is to protect his child and hence did his job precisely leaving Krishna with Yashoda and Nand.