Preheat oven to 220°C/200°C fan-forced. Spray 4 holes of a 1/3-cup capacity muffin pan with oil. Line holes with tortillas, crimping as required, to form a cup.
Spray a large non-stick frying pan with oil, then heat over medium heat. Cook onion, stirring, for 5 minutes or until softened. Add chicken and cook, stirring, for 5 minutes or until browned and just cooked through. Add paste and cook, stirring, for 1 minute or until combined. Remove pan from heat. Stir in corn and tomato until combined.
Spoon chicken mixture into tortilla cups and sprinkle with cheese. Bake for 12 minutes or until tortillas are golden and cheese has melted. Place 2 taco cups in each lunch box with yoghurt and watermelon.