Toast the English muffin halves until very lightly brown, 3 to 4 minutes.
Spoon 1/4 cup of the tomato sauce evenly over each English muffin half. Add any toppings you like (see Note).
Top each English muffin with a slice of mozzarella. Sprinkle over oregano, red pepper flakes, and kosher salt, as desired.
Bake or toast the pizzas until the cheese is melted and a bit bubbly, about 4 minutes. Serve, making sure your kids know that the pizzas are hot!
Notes
You can add anything to the pizzas: slivered cooked onions or peppers, sliced olives, chopped cooked broccoli, pepperoni, crumbled cooked sausage, and so on.