I googles plastic knives for childrens crafts and found information. It shows using 2 knives, crossing them into a X then decorating them. There appeared to be many more ideas.
Good luck

I don't know of anything with just knives; but if you take a foam block you can use the knives to push material into the block making various designs, like a quilt pattern. Just mark the design on the block and insert the edge of the material with the knife edge, butting the materials together.

I use lots of plastic knives for garden markers. Just write on them with Paint Pens. Sharpies and others are not permanent.For Sunday school, break one off and make crosses that can be decorated with odds and ends or tiny flowers. Maybe they could be used like Lincoln Logs and build with them by cutting into various lengths.