Hello Susan ! The nuance here is as such: "être descendu" can mean "to come down (from)" in the sense of leaving (something), such as "I came down from the attic" which implies leaving the attic. However, "avoir descendu" would be "to come down (something)", as in being on something and sliding/climbing/going down it. Therefore here to say "Jack came down the magic beanstalk." (i.e. progressed down its surface), you would say "Jacques A descendu le haricot magique.". I hope that's helpful! À bientôt !