change a apple id to a child id

There are 2 options I know of:

• Create a new child ID through Family Sharing. Of course, they would lose any content that they have from using the other Apple ID and email would be different.
• Add their current Apple ID to family sharing and use the Screen Time settings to restrict content however you want. You may even be able to change birthdate of the ID to make the restrictions easier. Here is a support article about changing the birthdate.

Update the date of birth associated with your Apple ID – Apple Support (UK)

I would try adding the current ID to see if you can make it work the way you want it to. Since the community pages here are unable to access or make changes to an Apple ID, here is a link directly to Apple Support that may help.

Contact - Official Apple Support

For as far as I know, you cannot concert an adult account in to a child account. See this link for further reference about child accounts

There are several articles in this forum about this. The only solution is to make a new Apple ID.

The section option in Mac Jim ID’s post will not work. As long as it isn’t created as a child ID, it will auto mark them as an adult.