I'd be tempted to count out (loudly) the calories in their lunch. For example
Ham& cheese sandwich - well that'll be about 500 calories, plus you bag of crisps,,about 100 calories, and that chocolate bar, 200 calories, oh- & don't forget that healthy fruit, 100 calories,many your can of coke 160 calories.
So, lets see, that's about 960 calories for your lunch - Blimey!