I've made lots of ornaments using styrofoam balls in any size. One is not a child-friendly activity - cover balls in various ribbons, using small pins to secure. You can get ribbons and trims at craft stores or fabric stores, and you can use short pins (not regular straight pins) which you put thru small beads, pearls or sequins to secure. Use a small loop of ribbon or fancy cord, inserted with a pin at the top, and attach a hanger hook.
Another easy and fun ornament is to take scraps of fabric - perhaps in Christmas colors, that you can find in the remnant bin at any fabric store. Place irregular scraps on a styrofoam ball, then use a regular flat screwdriver to press the edges into the ball. Just puncture the ball by pressing the screwdriver in about 1/4 inch from the fabric's edge. Then put a new piece next to it, pressing the new piece into the trench you created with the last one, and then pressing in the new edges. When you're done, the ball looks like it is quilted and puffy. Easy easy, and no mistakes possible - if you goof, just cover it up with a new piece of fabric. Use ribbon or a small piece of rick-rack to make a loop for hanging.
For your little one, try using foam Christmas shapes and glue, and covering containers. YOu can find inexpensive metal, wood and cardboard boxes/cans at craft stores, or get creative by spray-painting coffee cans or putting colored paper on oatmeal containers, then adding foam shapes. People can use them for displaying pine cones and evergreens (which you can provide), potpourri, Christmas cards, etc.
He might also enjoy rolling pine cones in peanut butter and then bird seed to make feeders for everyone. Messy but easy.