Maybe the Christian values that nick and kayla taught them while growing up were so strong, that not even they could fuck them up.
"I'm angry that dad is fucking a coke whore while mom is watching star trek reruns and drooling on the couch, but Wrath is deadly sin, so I can't act on it. God is testing me, I have to remain strong"
I could imagine the kids seeing their parents getting fucked up and losing everything that matters in life would remind them of the story of Job, and how they should not lose faith.
I could also be a retard