A fake knife can be a perfect accessory for dozens of different Halloween costumes, ranging from funny to scary. Your child could want to be a chef, a butcher or even a "cereal killer" who carries a Cheerios box with several fake knives stabbed into it. Using fake knives instead of real ones is obviously the way to go, both for your child's safety and your peace of mind. Also, your child is most likely prohibited from bringing a real knife to school.
Decide what you would like your knife to look like. If you have no idea, grab the nearest kitchen products catalogue for inspiration. Once you've picked a basic design, draw a profile of the blade and handle on your sheet of poster board. (If you're not artistically gifted, trace a knife from your kitchen. Also remember that the knife is fake and doesn't have to be completely realistic.) Cut out the shape with your scissors. This will be your template.
Collapse your cardboard box to make it completely flat. Trace your template onto one side of the box using your permanent marker. Cut out your knife shape with the scissors, or if the cardboard is too thick, an X-acto knife. Trace the template onto the box again, but only cut out the piece that represents the handle this time.
Place this new handle piece directly on top of the handle of your first cardboard piece so that they match up. Glue the two pieces together with your hot glue gun. Wrap your now thick handle entirely in black electrical tape. Use only one layer of tape so your knife doesn't get too bulky and difficult to hold. Check that none of the cardboard shows through.
Carefully wrap the blade end of the knife in aluminium foil, being certain not to crease or tear it. (The smoother your piece of foil is, the more realistic your knife will look.) Since you're not creasing the foil, it won't stick to itself well. Instead secure it with the glue gun, using as little glue as possible so there aren't any bumps.
Add extra electrical tape as needed if any cardboard shows between the knife handle and the blade.
Keep your knife shorter than 10" in length. If you make it too big it ends up looking more like a ninja sword. If your knife is meant to be a kitchen knife, paint three ½" grey circles on each side of the handle to represent the rivets.
Teach your child to keep his or her fake knife away from other childrens' faces, especially the eyes. Some places don't even allow fake weapons. Always check before sending the knife along with your child.