The healthiest breakfast cereals are the ones you make yourself from intact whole grains. Examples are oatmeal, barley, whole wheat, brown rice, etc. A large bowl of one of these topped with fresh fruit, berries, raisins, and some cinnamon are delicious.
My daughter and many kids like her grew up on these and enjoyed them.
Most dry cereals are high in salt and/or sugar and all of them are very dense in calories. Not the best choice at all.