I dont understand why Kevin gave in to peer pressure so easily he smokes in the season 2 episode Heart of Darkness some bad kid gives him a joint and instead of saying i dont want to smoke he smokes the joint, another episode is The Lake he meets a girl she gives him the joint and instead of saying i dont smoke he takes the joint and smokes it, their are other episodes where kevin is taken advantage of, do you think jack and norma would have been mad if they found out that kevin smoked ???

It was not a "joint" it was a cigarette. And considering how many young people smoked back in those days, it's actually a good and rare thing that Kevin only tried it twice.