1. First, prepare all ingredients. Put the plum tomatoes in a bowl; a child age 6 or older can snip them into rough pieces with kitchen shears as an adult steadies the bowl. Snip 1 cup basil leaves into small pieces. In a food processor, chop 1 medium onion and 4 cloves of garlic (a child can feed them in with supervision).
2. Heat 2 tbsp olive oil in a heavy saucepan over medium-low heat. A closely supervised 8 year old can add the onion and garlic, then stir constantly with a long wooden spoon for about 5 minutes. The onion and garlic are ready when they have softened, smell fragrant, and are just beginning to turn brown.
3. Add the chopped tomatoes and their juices, the basil, and the can of tomato paste to the pan, and give it a good stir. Bring the sauce to a gentle simmer. Let it cook, uncovered, for 30 minutes. Kids 8 and older can stir occasionally to keep the sauce from burning on the bottom.
4. In a small bowl, combine 1 tsp salt, 1 tsp freshly ground pepper, and for a spicier flavor, a healthy pinch of red-pepper flakes. Add the spices to the sauce after it is done cooking.