Changing password for child's iCloud account attached to parent's Apple ID

I’m looking to see if anyone knows how to get into a child’s account attached to the managing persons iCloud so when I go to Apple and I cloud it gives me the option to change the child’s account password..how can it be changed if it’s not known and if I can not reset online how do I get back in the account.. Any ideas🤔

[Re-Titled by Moderator]

Original Title: Password and Child Account