Is it disbelief to seek help from a living human in a matter beyond his capability—such as calling out to him from afar where he cannot hear, or an example of that being scaring children with a policeman?

Your statement that seeking aid from a human in matters beyond their capability is polytheism is correct. However, the example of a parent invoking a policeman to frighten a child is not related to seeking aid, because the invoker knows that the policeman will not prevent the child, but rather the purpose is merely to instill fear.