Is there a safe number of calories for kids on the gluten-free casein-free diet?

Sure, same as others: Calorie intake depends on age, sex, activity. Same calorie intake as if not on a restricted diet. See a dietician, given the above restrictions, to make sure no deficiencies of vitamins and minerals occur. Can be done, but more challenging. The more restricted the diet, the easier for imbalance (old insight from hippocrates-still true).