Hello lisl50,
To help us better assist you, can you let us know more about the error you're receiving? Do you receive a specific message every time you try to create a Child Account? If so, what does the exact error say?
The above details will help us plan how to proceed.
Please also check out the information in this link to ensure the correct process is being used, so we can make sure there aren't any issues there: Create an Apple ID for your child
In iOS 15 or iPadOS 15 or earlier
1. Go to Settings.
2. Tap your name, then tap Family Sharing.
3. Tap Add Member.
4. Tap Create an Account for a Child, then tap Continue.
5. Follow the onscreen instructions to finish setting up the account. You can use the child's email address for their Apple ID, or their Game Center nickname. Be sure to enter the correct birth date — you can't change it later.
Thanks!