Besides, in the real world parents of people who do horrible things have been known to hold out hope their child can be redeemed someday somehow. To an outside observer this may seem irrational or unrealistic, but as loving parents it stands to reason Han and Leia would hold out hope, no matter how faint.
In fact, that's what I like about Han's death scene. After a lifetime of being a hardened smuggler living among vile gangsters, shady bounty hunters, fighting to restore freedom to the galaxy and learning to shoot first and ask questions later no matter what the special editions try to tell us, Han shouldn't be an easy guy to catch with his guard down. So it stands to reason the only person he would lower his guard around is his estranged son who he is trying to reconcile with. Ren apparently knew this and played Han, thus becoming the only person who can truly swindle the master swindler.
In fact, that's what I like about Han's death scene. After a lifetime of being a hardened smuggler living among vile gangsters, shady bounty hunters, fighting to restore freedom to the galaxy and learning to shoot first and ask questions later no matter what the special editions try to tell us, Han shouldn't be an easy guy to catch with his guard down. So it stands to reason the only person he would lower his guard around is his estranged son who he is trying to reconcile with. Ren apparently knew this and played Han, thus becoming the only person who can truly swindle the master swindler.