Hi,
Thank you for posting your query in Microsoft Community.
I understand your concern, I will be glad to assist you with this.
I would suggest you to try the steps below and check if it helps:
1. Press Windows key + I to open settings.
2. Click "accounts".
3. Click "family and other users" on the left pane.
4. Click on the adult account and click "change account type".
5. Check if you have the option to change it to child.
However, please note that if the adult account's date of birth is set to exceed the age of 18 during the setup, the account will not be able to be monitored or controlled under Family Safety.
You will have to create a new child account.
For more information on how to add a child account, refer to the link below:
Set up family features
For more information on family safety, refer to the link below:
Family Safety: Frequently Asked Questions
Let us know if you need further assistance. We will be glad to help.