I don't care how sheltered you are, no mentally healthy adolescent naturally gets sexually curious about 5 year olds. It should be pretty obvious even to an idiot that a small child and a sexually mature girl are not the same physically. The "sheltered and curious" argument might have held some water if he was accused of peeping at his essentially same-age siblings, but not the little ones. Besides that, scriptural lessons would have made it very clear by that point that such behavior was totally beyond the pale.* Josh knew it was more than just typical adolescent lust, and JB knew it, too; but he was embarrassed at the loss of his vaunted control over everyone in the family. These days what I think JB is most angry about is that Josh's carelessness blew up a situation that JB had come to believe had been successfully buried for good.
(*Edited because my original wording probably was too religion-specific.)