Under certain age criteria you must use Family Sharing to even set up an Apple ID for a child.
Family Sharing and Apple ID for your child - https://support.apple.com/HT201084
If she already has an Apple ID (and it isn't something that may have special criteria such as a school ID) then you can try simply inviting her to join your family sharing group.
Click on this link to learn how to 'Set up Family Sharing' --> https://support.apple.com/HT201088
Read the section: "Invite people to join your family"