The young ones are the easiest ones to teach singing lessons to. Notice how a child can sing on tune in nursery rhymes and eventually on their own favorite songs as they get older. If your children have what it takes, and with their own willingness, you can enroll your child in singing lessons. The readiness of your child will greatly affect the whole teaching process. It is said that the most ideal age to start learning singing is between ages 5-10.
Please be aware though that their voice is still immature and fragile and more prone to vocal strain which often leads to damage and voice hoarseness. The goal to start at an earlier age is to better introduce the good singing techniques that can be polished as time goes by.