I had a piece of extra-thick craft foam (it comes in individual sheets in the foam section at the craft store) and just drew simple shapes on the foam, then cut them out with an x-acto knife (scissors mashed the edge down a bit and made a less-crisp image). The tree, ornaments, and star are each separate stamps that Little Flower drew and I cut out. I then mounted them on a piece of matboard and set the kids to stamping! They worked well with a stamp pad but that limited our choices, so the kids colored them with markers then stamped. I liked the sort of textured effect this gave.
Next up, we need to get a big roll of butcher paper to stamp up some Christmas wrap!