I don't know for a fact about the crossing-water thing, but in "Psychic Self-Defense", the author says he's experienced that many times, e.g. that an entity (he calls them "negs" from Negative Entities) attached itself to someone briefly, but couldn't follow that person over a water pipe. He says, for example, that he had good results with little kids getting those "night terrors" (he attributes them to entities harassing the children). He says he picked up the kid and took it outside, crossing a water pipe or even a hose they let water flow through, and the child immediately stopped screaming and was fine afterwards. If it gets really bad, he says, you can put them in the shower, too.
That's another topic: I firmly believe that, when children talk about monsters in their room and so forth, that it should be taken seriously by the parents. I've talked to many people about supernatural childhood experiences, and even skeptics often had impressive stories to tell. One guy I knew told me that his first childhood memory was being a baby or toddler lying in his crib and shadowy hands came out of the walls, grabbing for him. Another friend of mine started to experience severe astral attacks when she was 10 or 12. Same thing, shadowy things would come out of the walls, bearing down on her, trying to suffocate her. Myself, I was quite psychic as a kid already, and I always knew there were "things" in my room. Sometimes they would play tricks on me, like hiding things. For example, I would leave some color pencils on my desk, and in the morning find them on top of my bookshelf. Since I was never a sleepwalker (my parents would have noticed), I'm quite sure that I didn't do it myself. Children are much more sensitive to energies and entities than we are. Sometimes I hear my little 18-month-old having lively (but of course unintelligible) conservations in his room over the baby monitor. Maybe he IS talking to someone--how would I know?