Why can’t I log in to my child’s account. It keeps saying wrong password when I know I’m putting the correct password and now the account is locked for 12 days

Try changing the password to see if that helps.

If you know the password.

 Apple ID Password Change.   -  

ASC YouTube for How to reset your Apple ID password

 What to do after you change your Apple ID or password - Apple Support  -   I suggest doing this before you change your Apple ID or password.

If you forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if you can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.

Apple ID - Manage

Apple ID – Forgot Password  *      

You don't have to answer me, but consider these questions.

How do you know that you have the correct password? How do you know that your child has not changed it against your will? Could it be that your child learned his/her Apple ID's password and changed it so as to escape Screen Time restrictions that you had imposed?

Perhaps none of this is true, but it is what popped into my mind when reading your story.