Basically, whatever they'll eat...it's the calories that count.
Bagels and peanut butter were our standard, or tortillas and PB to save space. String cheese usually goes over well, too.
Nathan and I have been backpacking since he was 5. (He's 18 and off to college this fall, so I lose my pack horse now that he's actually useful...)
When he was younger, he wouldn't touch jerky or sports bars. It was PB, cheese, some sort of bread, yogurt pretzels and red Gatorade for him. The Gatorade (Powerade, etc.) was actually a critical item because he'd lose enough electrolytes to start having leg cramps. We found that the Reliv
"Innergize" sports drink that is blended for kids ("cool punch" or red flavor) actually worked better than Gatorade because it added calcium which he seemed to need when hiking.
Once he hit about 14 or 15, he couldn't afford to be picky and would eat whatever you put in front of him...funny how kids tastes broaden when they're desperate for calories!!