I actually prefer using larger diameter rounded neodymium (rare earth) magnets (avoids getting caught via inadvertent edge snagging) backed by nano-technology double-sided tape (marketed as “alien tape”). This option can be custom sized (up or down) to suit the particular knife you want to secure, winds up being a whole lot less expensive than most of the special purpose gun mounts I've tried, and the cushioning effect of the thicker mounting tape helps keep from causing the magnet to shatter if a metallic blade (or other object) is aggressively seated. I also prefer mounting on objects that do not conduct magnetic force (such as wood), so that I don't accidentally “wipe” any hard drives/memory devices that may come into near proximity of mounting magnets. My latest “fail” on a magnetic gun mount was one that was designed to allow you to rack the pistol slide while in the magnetic holder. Turns out it wouldn't fit because combination day glo/tritium night sight tube body was too long to fit the housing notch cut out from the main mount body. Unfortunately the device design also renders it of very limited benefit even when used with a knife because the magnetic portion is in a central channel that is well below a surrounding and non-metallic body.
While not directly magnetic, I'm a tactical fan of staging non-metallic knives such a the Cold Steel brand zytel or grivory synthetic material compositions in potentially wet environments – especially in bathrooms. If not already equipped with a metal keyring (like the cruciform dagger design) that will hang from a magnet, you could always drill a shallow hole in the weapon, glue in its' own circular rare earth magnet and have a strong double attraction factor working for you (just mind the pole orientation so the magnets don't wind up repelling each other). Such synthetic blades obviously are not designed to be re-sharpened, but for single-use defensive deployments – they are capable of producing formidable penetrating injuries in attackers.
I think you're absolutely spot-on when making the point that the majority of such arrangements are not suitable if minors will be inside your home. In addition to the safety concerns posed by younger children, you could face potential legal jeopardy if teen-aged visitors could gain unrestricted access to any style weaponry in your home. I am reminded that laws in many states treat knives and clubs as being even more “heinous” than firearms.
Anyone have experience using ceramic bladed knives for defensive purposes? Beyond those styles/applications mentioned by Ed Calderon, most of the ceramic blades I've seen appear to me to be too thin and brittle to make them good candidates for defensive work; however, I always remain happy to be educated on any new developments!
Stay “sharp” and frosty my fellow members!