That is a completely different issue, a health related issue. Most children I know do fine with milk.
Those calories still go into the total calories. it isn't digestion that is the issue as much as you burn 4 calories by simply walking for a minute or so. But that is true for no matter where the calories are coming from. They are eating carrots, celery and apples for the many nutrients that those foods provide.
Nutrition Facts and Analysis for Apples, raw, with skin [Includes USDA commodity food A343]
Nutrition Facts and Analysis for Carrots, raw [Includes USDA commodity food A099]
Nutrition Facts and Analysis for Celery, raw
And those are just the very base facts. Other pages talk about the additional health benefits from each of these foods.
That lunch does cut it for a 7 or 8 year old, and certainly younger children, and that is who it is being fed to. Especially when, as I've said multiple times (since I am a mother and know what is going on in at least two different schools just this past year), they also have snack times for elementary school kids that provide more calories as well.