changing family share member to child

See the Apple Support article(s) below.

Apple ID - Update the birth date associated with.       

1. To update the birthdate, you need to be signed in to the Apple ID you wish to update. Easiest thing to do, is to log in to https://appleid.apple.com with the child Apple ID, and follow the steps in the link provided by Eric.
2. You can send the invite before or after you change the birthdate. However if you want to change the birthdate to an age younger than 13, then you need to invite the Apple ID to family sharing first.You still need to log in to the Apple ID directly to change the birthdate however. Apple IDs with an age set to under 13 years old must be part of a Family Group. They cannot exist on their own. So before you change the birthdate to something under 13, the Apple ID needs to already be part of a family group.
3. This step will happen if and only if the Apple ID’s age is already set to under 13 years of age, and as such the Apple ID is part of a Family Group already. Since the Apple ID is an adult, this step is not required. The adult Apple ID accepts the invite on its own.

The only thing that determines an adult from a child in family sharing is the age / birth date set for the Apple ID that was invited.

If you want to change the age / birth date of the child's Apple ID to something under 13 years of age, you will need to invite the Apple ID to Family Sharing first, and then change the birth date as explained in the link provided by Eric above.

Do note that the only thing the Apple ID age affects is the ability to turn on Ask to Buy for store purchases. Ask to buy cannot be turned on on Apple IDs that have an age over 18 years old.

All other age based restrictions are assigned per device under Screen Time but have no relation to the age set for the Apple ID.