Are they eating a large or late snack in the afternoon? I am surprised that both children are not very hungry at dinnertime. If you were to give a snack 1-hour before dinner, I am concerned that it would further reduce their appetite for dinner.
I would offer a reasonable serving size at dinner. Children should not generally have to clean their plates. However, if you do not think that they ate reasonably, then dessert should be withheld. Dessert can be given later in the evening as a small bedtime snack if they ate reasonably for dinner.
From a convenience standpoint for you, I would discourage having dinner served in two halves.
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