As long as they are heating heathily and in the vcorrect proportions of food, i would let them eat 'til they are full. Small children rarely overeat and will only be overweight if they are eating the wrong sort of food.
Overeating is a learned thing.
If my kids are hungry after they have finished their dinner and there are no leftovers, then they can have a banana or a piece of bread and butter. The only things that should be limited are sweets, chocolates, deserts, crisps etc.
Small children burn up an enormous amount of calories and if these are not being replaced, then I believe that a child will not grow to their potential.
Children are never encouraged to lose weight even if they are overweight, they are put on a normal healthy diet and then they grow into their weight.