I have a 12 yr old daughter and a 14 yr old son. I try to cook them decent food but they have access to their pocket money and quite a bit of it goes on junk. Calorie wise it is hard to see where they stash it all as they are still quite slim.
For my 7 year old daughter it really depends on the day. But some things are consistent. She MUST eat immediately upon waking or she becomes very irritable and crabby. She generally grabs a piece of fruit, a bell pepper, or carrots as we head out the door to walk the dog as soon as we wake up. When we come back from walking 2 miles she may eat anywhere from 2-8 slices of bacon, 2-3 eggs, and possibly some broccoli and cauliflower, some raw cacao with a tablespoon of cream off the top of the can of coconut milk. If she isn't in the mood for bacon and eggs she may drink a fruit smoothie.
She will grab some fruit later in the morning. Lunch varies I pack sausage or beef jerky or left over dinner, vegetables, goat cheese, salami etc... she may eat half a pound of raw carrots several days in a row. She some times eats an awesome dinner of meat, vegetables, and then a "sweet" treat consisting of some type of cookie or cupcake made with almonds or coconut flour and very little of the sweet. A favorite is canned pumpkin with cinnamon and a splash of maple syrup.
Other nights she may eat a few bites and then nothing. She swims 5 days a week and when she swims in the evening she often eats her dinner during the car ride home. She may have a package of beef jerky, goat cheese, some macadamia nuts etc...
But it varies day by day so I pretty much just keep a well stocked lunch box with us when we leave for the day (we are homeschoolers) so we are out and about most days. She is muscular and looks relatively slender compared to other kids I see. I think she is about 125 cm or so and 50 pounds.