Sri Krishna had many sons but I am unable to find any reference of his sons in Mahabharata war. So where were they during the war?
You must acknowledge the fact that Shri Krishna, had vowed not to participate in the war as a warrior. He had confirmed to Duryodhana and Arjuna, who had visited Lord Krishna, to request him to join their sides, that he could not choose between the two, because they both were equally dear to him. So he would split his army which would fight, on one side and himself who would not pick a weapon on another side. And it was up to them to choose between him and his army. Duryodhana chose the army and Arjuna chose Shri Krishna.
Now when the father vows not to pick a weapon, how could his son be expected to pick a weapon and take sides?