My DS is 8, totally average height and is slim. He eats:
Grapes and half an apple, couple of tablespoons of Greek yogurt with maple syrup for breakfast.
Tiny pot of strawberries for fruit break
Morning tea is a cheese stick, pot of grapes and small bag of pretzels
Lunch - cheese sandwich, biscuit, half a bag of popcorn, cucumber sticks.
After school - slice of bread and butter if he’s got an activity, otherwise I do dinner at 5 which is normally 3/4 adult size of whatever we have (normally meat, veg, potatoes, stir fry, steak, pasta, something simple like that)
Ice cream for pudding plus some fruit and maybe a bag of rice crackers.
He’s active - runs around at school and also swims twice a week, plays tennis for 3-4 hours per week, does two dance classes etc so is slim.