That town really needs to put a lock on the ladder of it's water tower. If it's that easy for an mentally handicapped teen to get up it, it's hardly his family's fault.
YMMV, with some older water towers, the most that can be done short of 24 hour guard is have a fence. It's not like there's a door to lock the ladder behind, and to a person as mentally handicapped as Arnie, signs for security systems and cameras mean little to nothing. With the central location of the water tower a loud alarm would be a nuisance. Their town doesn't seem to have much going for it, like the kind of place that if it wasn't for Arnie and his poor impulse control towards climbing the water tower, anything other than a normal chain link fence would be excessive.
In the film's opening monologue, Gilbert says that his youngest sister Ellen just turned fifteen. Yet four sentences later, he states his father died seventeen years ago. So who's Ellen's dad?