ATMachine said:

For my money, by far the most intriguing deleted subplot in that movie involves the lost scenes where it's revealed that Khan has an infant son of his own, whom he quite willingly blows up along with the Genesis Device in his last attempt at revenge on Kirk.

It's not supposed to be Khan's son specifically, just a member of his crew's child. Chekov found his crib in the containers, then we see him wandering around the Genesis device before it goes off. Who lets a toddler into a transporter room? What does that add to the movie? It makes a triumphant moment actually very sad, so it was right to be cut.