By default the family organiser’s Apple ID is the rescue email address for a child’s Apple ID. You should be able to reset the password for your child’s Apple ID at iforgot.apple.com. I believe you will need the security question answers.
After that, child Apple IDs can be:
Moved to another family group
Deleted via an escalation through Apple support (officially they can’t be deleted at all until the owner is 13, however some people have had success through Apple support).
But they cannot be:
Removed from a family group (unless being moved to a new group, as above)
Deleted without the assistance of Apple support.
Similarly, you cannot disband your family account without first getting rid of the child Apple ID in one of the above ways.