"A kindly tongue is the lodestone of the hearts of men. It is the bread of the spirit, it clotheth the words with meaning, it is the fountain of the light of wisdom and understanding." - Baha'u'llah, Epistle to the Son of the Wolf
Observing children and adults in conversation, I notice how often eye contact is made, how often the words 'please' and 'thank you' are used, and how often kindly words shape and display how they respect one another. At the earliest of ages, children can begin the habit of using 'please' and 'thank you' to demonstrate respect, an essential virtue for any relationship - especially those within a family.
For a child to develop this simple way of showing respect, the parents need to model it. By using these words from parent to parent and from parent to child, we create a normal pattern of respect for all. Now, interestingly, many of us miss our opportunities to use these words. Humbling oneself by asking 'please' and showing gratitude for the smallest of things may not be in our own normal pattern of behavior. Family members may take each other's actions for granted and conversations may be stripped of these foundational words.
Begin, if you don't already, recognizing how often you use these words with your spouse or loved one. Use them with your children and kindly encourage them to use them with you. If your child asks for something without ending or beginning with 'please', then, with a kind tone, ask, "and...." Give them opportunities without reminding them. Choose when you remind them wisely. When they are quite young, a toddler, it can be more often. As they get older, less often.
Never respond to a child that demands or orders something from you. You'll know when their tone is inappropriate or not respectful. Of course, with children they need to learn that the tone is inappropriate, so this is your moment to kindly teach them. But how might you teach them? Let's consider these words from Abdu'l-Baha...
"Let not your heart be offended with anyone. If some one commits an error and wrong toward you, you must instantly forgive him. Do not complain of others. Refrain from reprimanding them, and if you wish to give admonition or advice, let it be offered in such a way that it will not burden the bearer. Turn all your thoughts toward bringing joy to hearts." - The Promulgation of Universal Peace
How does this apply to your own child? I do not think that this means we should use the teachable moment and make light of it and smile. Perhaps, we merely shouldn't make it heavy, as if the world crashed down on his/her shoulders. Step-by-step, 'please'-by-'thank you' - respect for family, respect for others, and respect for self begins.