My understanding is if you are creating a minor child (not just any offspring of any age) account this can only be done if they are then part of a family group. In other words, creating an ID for them automatically adds them to the group. Do you not see them in your family list?
Create an Apple ID for your child - Apple Support
This document has an image of a family list on an iPhone --> Set up Family Sharing on iPhone --> Set up Family Sharing on iPhone - Apple Support
Go to Settings > [your name] > Family Sharing
If that doesn't work it sometimes helps get your device to take a fresh look at settings:
Try signing out of your account and back in again.
Try restarting your device.
Contact Apple and ask if they can tell you what this means, particularly if you see it repeatedly over a day or so:
Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
"See a list of Apple phone numbers around the world."
Click here --> Contact Apple for support and service - Apple Support