why do crash and mike feel challenged to play tricks on penn in seventh grade

I think this is true to a point. Penn is the new neighbor kid. With no siblings and few toys, Penn is a curiosity to Crash. He's curious and easy to impress. He's memorized a bunch of random knowledge and converses pretty easily with adults. When he hears about Scooter's stroke, Penn stops by to encourage the old man. He gives him his most prized possession -- the can of dirt from his great grandpa -- as a boon of good will. In the end, the gesture is so compelling that it persuades Crash to give Penn a second chance. Turns out he's just a sweet guy who doesn't feel the need to compete.