He's also more aware of how worried his parents would get if he was out for too long. He's also neurotic enough to know he'll be in the library for a while until the storm ends - he might catch a cold if he goes out before the rain has definitely cleared up.
Makes you wonder why they didn't or if they couldn't get LeVar Burton a role for the movie, given that he is a strong advocate of reading (and hosts Reading Rainbow to boot). This movie would be right up his alley, plus his colleagues from TNG got roles for the film.
Adventure is a swashbuckling, courageous type, exemplary of the daring and bravado his genre. Fantasy is a fairy godmother sort, exemplary of the magic and wonder of her genre. So why is Horror a coward? Shouldn't Horror be a master of fear and fright, exemplary of the terror his genre seeks to evoke? ...but then, I guess that doesn't work for a kid's movie.