Show your child how to scrub the potatoes using a vegetable or mushroom brush and water. In assembly-line fashion, once your child has successfully scrubbed one of the potatoes, he or she can hand it to you and you can cut it in half widthwise. Then place each potato half, cut side down, on a cutting board. Use an apple cutter or corer (pushing down from the top of the potato) or a knife to make wedge steak fries (older children may be able to do this themselves under supervision). Soak the potato wedges in water if time permits to remove excess starch (about an hour). Dry them well.
Preheat oven to 425°F. Pour the oil into the bottom of a 9" x 13" baking pan. Let your child spread the oil evenly over the bottom of the pan using a pastry brush. Place the potato wedges in the pan. Spray the tops of the fries generously with nonstick cooking spray. Bake the fries for about 20 minutes. Flip the fries over and sprinkle with salt or seasoning salt if desired. Bake 10 minutes more.