a doubt in mahabharata

In some remote village of India, one masterji is teaching the Mahabharata
Story to class 6 students.
He is at the 'Krishna janma' part of it.
Masterji: "Kamsa heard the Aakaashwani that his sister's 8th child is
going to kill him.
He was furious. He ordered to put vasudev n devki behind the bars.

First Son is born, and kamsa kills him by poisoning...

Second one is born n kamsa throws him off the mountain peak.

Third one is born .....

Now Ramu, who is smartest of the lot, puts up his hand.

"Masterji, I have a doubt" (sounding nervous n confused)
Masterji: "Ramu beta, whole India does not have doubt in Mahabharata

Then how come u have one?"

Ramu: "Masterji, if Kamsa knew that Devaki's 8th child was going to kill

him, WHY THE HELL DID HE PUT VASUDEV AND DEVAKI IN THE SAME CELL? "