String cheese: It's a quick source of protein, very easy to stick in your child's bag.
Dried fruit: Berries, apples, apricots, etc. are filled with natural sugars, and deliver a good amount of fibers.
Pop corn: If made naturally, not cooked with loads of sugar and oil, it's a low-calorie snack.
Whole grain crackers with cheese: The whole grain crackers will give protein and the feeling of being full longer.
Baked vegetable chips: Very easy to make, and low in calories. Better than actual chips.