Change child under 13 password?

Do you perhaps have a parental control set to inhibit the ability of the child to change it from their device ?

Excerpt:

Here are some features and settings that you can allow changes for:

• Passcode Changes: Prevent changes to your passcode
• Account Changes: Prevent account changes in Accounts & Passwords

See:

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

A quick C&P from the linked article in previous reply:

You can allow changes to other settings and features, the same way you can allow changes to privacy settings.

1. Go to Settings and tap Screen Time.
2. Tap Content & Privacy Restrictions. If asked, enter your passcode.
3. Under Allow Changes, select the features or settings that you want to allow changes to and choose Allow or Don't Allow. 

Create an Apple ID for your child - Apple Support

See this link and follow the instructions provided