You may, at your discretion, allow a minor child, that you are the parent or legal guardian of, access to your account. They can essentially play on the WoW account you are registered to, if you wish.
If you want to give them their own WoW account to play on, you could add it to your current Battle.net account. I would recommend that you create a separate Battle.net account, under your name, that they’ll use. This allows you to set parental controls on it, that won’t impact your ability to play on your account, and give them their own account for their own pet, mount, toy, heirloom, achievement, etc… collection.
If they are over 13 but under 18, they can have an account in their name, but you’ll be required to set up parental controls.