When Jon pardon the Umber and Karstaks kids, I remember something Tywin said: When your enemies defy you, you must serve them steel and fire. When they go to their knees, however, you must help them back to their feet. Elsewise no man will ever bend the knee to you.
He is already King of the North, but he need all the help possible. Showing mercy to their children’s enemies could encourage other Houses to lend their hands in the wars to come. Also, he was very enfatic in saying that he would not remove the kids from their ancestral homes, which is very important for two reasons: first, the Lords who fought with him lend their sword so the Starks could take back their home; and second, respecting nobility rights is a very important characheristic when dealing with a room full of nobles.