Learn the strict difference in French between formal and informal address. Adults other than family are addressed in a more formal, polite manner than family members, close friends and children.
The formal means of address in French is based on use of the polite or plural "vous" form of the word "you." In French this is called "parler vouvoiement," meaning "to speak using the vous form."
The less formal address is based on use of the "tu" form of "you." This is called "parler tutoiement" in French, meaning "to speak using the tu form."
In both cases you must know how to conjugate your verb so it corresponds to the form of address. You can check how to do this for most verbs using a book of French verb tables or an online source, such as the one in the resources section below.
To address one child always use the "tu" form, regardless of whether the child is your own or not. For example to ask a child how old she is, you would say, "Quelle age as tu?" To ask a child if he would like an apple, you would ask, "Tu veux une pomme?" To ask a child if she enjoys school, you would ask, "Tu aimes l'ecole?"
To address more than one child, you address them collectively using the "vous" form. For example, to ask a group of children if they would like a snack, you would say, "Vous voulez quelque chose a manger?" To tell a class of children they must speak to you using the formal tense, you would say, "Vous devez parler vouvoiement!"