Ask the kid to come up with its own coat of arms. When the sketch is ready, transfer the image onto thin cardboard and cut out. You get the stencil on a sheet of cardboard.
The stencil should not be too large - no more than 10x10 see
Show the child how to work with it. You can outline its contours with a pencil or marker, placing the stencil on the paper and paint, paint, using a pad of foam rubber or matter. The baby can now do many of the same pictures. Ask him to use a stencil for the bookplates of children's books.