You need to create an Apple ID for your child. After you create the Apple ID you can turn use Family Sharing to allow that Apple ID on a device that you aren’t using. IPads are not designed to be used by multiple users, they are designed to be used for one user only so if you turn on Family Sharing, that has to be used for separate devices. Each device will need to have it’s own Apple ID. If you are trying to set it up for your child on a device that you use, then it’s always going to have access to everything that you do on it as that is how it is designed.