From early childhood, as soon as the child learns to speak a little, teach it to use the magic words: "please", "thank you." Explain when it is necessary to use them. Adults should not do without these words when communicating with the child. You need all the time to remind your child that he did not forget. We can agree that, if he asks you about something without the word "please", you are not going to fulfill his request. And if he says the magic word, then the request will be executed. Try not to change this rule (of course, if the desires of the children don't go beyond the boundaries of reproduction).