reset my child's icloud password

Hmm... that information recently vanished from Apple's documentation. See if the information in the document in the next section still applies. Sometimes Apple removes things from documentation but the features still exist undocumented. Unfortunately the document no longer includes the special instructions for doing this as an adult for a child and simply refers you to this document which does not have special instructions for a child's account: If you forgot your Apple ID password --> https://support.apple.com/HT201487

Create an Apple ID for your child - https://support.apple.com/HT201084

- "Your email address is the rescue email address for your child’s account, and you can use it with the security questions you provided to reset a forgotten password. You can keep your your child's account more secure with two-factor authentication."

- "If you need to see the options you have for managing your child's account, you can sign in with their Apple ID and password at https://privacy.apple.com/account ."