Whisk together eggs, milk, salt and pepper, to taste; set aside. Melt butter or margarine in a skillet over medium heat. Add tomato, peppers and chili powder (if using) and saute until peppers are tender, about 5 minutes. Pour egg mixture into the skillet and cook, stirring gently, until eggs are just set, about 5 minutes. Divide egg mixture equally over the tostada shells. Place cheese, salsa and shredded lettuce in separate bowls so that kids can customize their meal by adding the toppings they like.