The are many possible responses. One of the most common when someone calls to you is: "What?" meaning "What do you want?" Another is "Yes?"or "yeah?" meaning "I hear you. What do you want?" Some people say just "huh?" with a sort of nasal sound on the "u."

In American English from the midwest we would respond with "Yes?" or "Hi!" or some other greeting in either situation. For your child we would respond "HI, Honey!" or "Hi Dear!" or use a nickname or pet name for the child, such as, "Hi, Pumpkin!" or a personalized peculiar name such as "Hi, Michael boy!" or even more as my sister Lori would mis-pronounce her name as something like "Ouie", so, our dad would say, "Yes, Ouie?" or "Hi there, Ouie!" with joy.

他们两个都说得对了，　对这样问题美国人有很多不同的回答,　真的没有最好的，　就是你想怎么答理就怎么答理！