Last year my son was Chucky from Child's Play! I made the entire costume for him and the last thing I did was construct his weapon. I actually tried finding an actual prop weapon small enough to look proportionate to his body but wasn't able to. I needed something small and soft enough that it couldn't hurt him either. I grabbed some thick craft foam I had and went to work!
I just free-handed these and cut them out. I use metallic paint and regular acrylic paint for the symbols. The finished piece look great and more realistic than I had imagined. We just used these for photos so he wasn't carrying it around the entire night. Notice the paint coming off of the knife handle. I didn't seal it or anything at that point and I did end up making two along with a mini-hammer!
And here's a pic showing my crazy Halloween mess I make each year with my cat in the background. Haha!