When trying to add a new child email, after signing in with my account to authenticate, I...

Hi! I'm Rostom, an Independent Advisor. I will be glad to help you here. No worries, we will do our best now to get this sorted out.

Could you please try to use a different device or internet browser to finish the setup?

it should give you an option for more than one way to verify that security information

try the security check again click the option other ways to verify

and click email or Microsoft authenticator

then you can receive the verification via email or you can sign in using the authenticator app

then you should be able to add a new phone number to your account

If you are playing on two consoles he will need his own account.

Create a child account and add family members on Xbox One.

• Press the Xbox button  on your controller to open the guide.
  
• Select Profile & system, and sign in to your account.
  
• Press the Xbox button  again to open the guide, and select Profile & system > Settings > Account.
  
• Select Family settings > Manage family members.
  
• Select Add to family.
  
• Select Add new.
  
• When the child is prompted to enter an email address, enter their existing email or, if they don’t have one, select Get a new email.
  
• Create a new @outlook.com email address that your child can use.
  
• Enter their first name, and then their last name.
  

The following link has more information about a child account:

Xbox Support Help Page - Child Account

This is not helpful as I've already stated that this method is outdated and you cannot change your child's email address while signed into your own account. There is no place to change it. The only method is to sign into Microsoft with your child's login information and add an alias or second email address. But the parent cannot do this from their account. The problem is that if your child no longer has access to their old email address (hence the reason for changing it), you cannot confirm the login as Microsoft requires you to authenticate the login attempt with a code. If you don't have the email address, you cannot receive the code. I'm not sure why the brains at Microsoft haven't thought of a workaround for this and instead, my kid lost two years of game play from their account.

Hi,

just add the two that aren't on the system by signing in using their email and password.

Scroll left at home screen -> profiles -> add and manage -> add new -> Sign in using the email and password you used to sign up the child accounts you need to do this twice.

note: don't press A on get a new account.

support.xbox.com/.../sign-in-to-another-console

Have a great one