Here's the extract:
— Mais enfin, p’tit gars, il lui a dit, le chef, il ne faut pas avoir peur.
— Si, il faut! a crié Paulin. Si, il faut!
Translating it into English in my head, it's a pretty funny exchange:
"But kiddo," the leader told him. "You don't have to be afraid!"
"Yes I do!" yelled Paulin. "Yes I do!"
But that's because "Yes I do" is an odd answer to "You don't have to be afraid." It's something a little kid would say.
But "il faut" is idiomatic, and if I translate the idiom differently, it's not funny, or at least not as funny:
"But kiddo," the leader told him. "There's nothing to be afraid of."
"Yes there is!" yelled Paulin. "Yes there is!"
I know humor is subjective, but subjectively speaking, in this context is "Si, il faut!" normal everyday French, or a somewhat silly phrasing that a kid would use?