Jimmy had already inspected the house, and clearly, Father Koch must have instructed those two children. They only cleaned the part of the house they lived in, leaving the rest untouched. Of course, it could also be that the two kids were too lazy and couldn't be bothered with any place other than where they slept.
Jimmy was quite concerned, after all, traveling to another world is not a trivial matter. If those two kids were reckless, there's no telling what could happen. Unfortunately, Roland valued this door highly and did not provide any activation method, only saying that without the corresponding code, it was impossible to go there. But even an incorrect code carries some risks, doesn't it?
