1. In a large skillet, heat oil over moderate heat until warm. Add the onion, half the oregano, and the cumin; sauté for 5 minutes.
2. Add the ground meat and cook 5 minutes, stirring well to incorporate all the ingredients. Stir in the tomato paste, salt, pepper, salsa, water, and remaining oregano; cook another 6 minutes. (The meat mixture can be cooked several hours ahead of time and refrigerated. Before serving, reheat in a skillet over low heat. If the meat seems dry, add 2 tablespoons of water while reheating.)
3. To serve: Wrap tortillas in tin foil and warm them in a 200º oven for about 4 minutes. Divide the meat mixture among the 10 tortillas. Place the desired accompaniments in small bowls and let each child fill and fold to create his own custom taco.
How kids can help: Pick fillings and set them out in bowls.