Taking the cream out is (by some diet theories) bad. The fat in whole milk slows down the absorption of lactose, leading to a slower rise in blood glucose compared to skim milk. Whole milk is more satiating as well, because of the fat.

If you are trying to have some reasonable balance of fat, protein, and carbs in your diet, pushing kids from whole to skim milk is going to move the diet towards consuming more sugar/carbs, even if you have a seperate rule trying to tighten sugar consumption.

In my country the lowest fat milk has added lactose.

It did twenty years ago, when I noticed, I have not bought it since

For the milk you don't add sugar directly, but you end up adding more carbs to the rest of the meal when you take out nothing but fat from the milk.