feivel, if the child is restricted to certain cereals, then he is not being given whatever he likes. Adding fruit to the cereal will help balance his diet too. Some cereals also contain protein, or he can eat some egg, peanut butter, or fish for protein.
Whether the child in general eats enough is also relevant. I do think that if a child doesn’t eat much he should perhaps be given more latitude in his food choices if that will encourage him to eat more. (I may be projecting here, since my children are all underweight & tend to be picky eaters). Obviously if the child is overweight more restrictions may be appropriate.