As well as a printed blank hundred square, another way to visualise decimals is to make a decimal stick. Get a cane or long stick, place with tape a number 1 to the right hand end, then a 0 to the left. Ask them now to place 0.5 in the middle, then together work out where all the other decimal points should go.
If you make your own, you can just draw three circles on a page and then 2-5 triangles on a page, and ask the child to match each circle with a triangle by drawing a line from shape to shape. Vary the shapes and the amounts. Sometimes the amounts should be equal, sometimes not.