Hi Ss_stinger,
It sounds like you're wondering how to change the settings for their Touch ID and Passcode on your child's device. We're glad you've joined us in Apple Support Communities. Let's see if this helps.
It looks like you've had the chance to review If Touch ID isn't working on your iPhone or iPad. That's a great start, but we have some additional advice for you.
It seems like you might have parental controls and Screen Time set up for your child. To allow Passcode Changes, follow the steps in the section, "Allow changes to other settings and features" in Use parental controls on your child's iPhone, iPad, and iPod touch and set changes to "Allowed."
If the issue remains after following those steps, let us know what steps you had a chance to try from above.
We hope that helps with your question and take care!