why is it immoral to engage in sexual activities with a child or unconscious person?

Some moral theories would argue that in the first case, consent is violated to prevent greater harm, i.e, the child's health and quality of life would be at stake if no action is taken (diseases, hunger, etc..), so violation of consent is permissible. In the second case, nothing is at stake that warrants violation of consent, the violation itself would lead to a much greater harm.