I think that Joe is the leader  because,#1 He's the adult and the veteran Screep scewerer,it means someone who scewers, so he knows what to do and he's like chaperon. 

Senuor Jackis ofical cry baby and follower. He dosen't do much but complain.

Isadora is a sciencey scientist kid and generator of ideas.She had the idea to sit still so the veil trees would let them in