I want to set up a yahoo mail address for my child, but I don't know why it is asking me for a credit card! (She will never be on the internet without me.)

Yes, it's normal. They don't want a child setting up an account. Therefore, if when putting the age in, you put in an age under 18, they will ask for a credit card so they can verify that an adult is in the picture supervising the child. If you don't want this, put in an age 18 or above when setting up the account. It won't make any difference in the functioning of the account.