Trying to create a family group but eldest child has xbox in my email account name

Because the original account is now your eldest son's account. Technically he has to consent all the changes on the family settings.

I think, you and your youngest son should be removed from the family group first (the one that you share with your eldest son). He might have to do it, as he has the parental control.

Then sign in your xbox, and create a new family group for you and your youngest son.

Remove yourself or a family member from your family on Xbox One

Create a child account and add family members on Xbox One

Hope it helps and did not confuse you more, let us know if that works!

Hi. My name is Brilliant, an independent advisor. I am happy to assist you today.

Parent and child accounts on Xbox are designed to help parents manage their children’s gaming activities. A parent account is the primary account that can create and manage child accounts. Child accounts are created for children under the age of 18 and are linked to the parent account. The parent account can set up parental controls, such as screen time limits, content restrictions, and spending limits, for each child account.

You cannot change your account to be a child account. You will have to create a new account for your child.

To create a child account, the parent account must first create a family group. The family group can have up to six members, including the parent account. Once the family group is created, the parent account can add a child account to the group. The child account will be linked to the parent account and will have access to the parent account’s Xbox Live Gold subscription and other benefits.

I hope this helps.

Hello, My name is Jesse. I'm an Independent Advisor and a member of the Microsoft community and I'd love to assist you today.

Here's what I'll suggest;

Create the child's account Independently of yours first. Just go to Xbox.com and sign up the child's account.

Afterwards using your own account, go to account.microsoft.com/family and create a new family group. Invite the child account into that family group as a member.

Then you can control all the settings you need.

Hope this helps

it would be best to create a new child account for your son then change the date of birth for the account in your name.

you may need to create a new adult account for yourself to change the date of birth on the account in your name.

you would need to add this account to your family group to make the changes on your account then use either of those to set permissions on your son's account

this link has the info on changing the date of birth.

https://support.microsoft.com/en-us/help/12411/microsoft-account-change-birthdate

this link has the info on setting up a family group and adding members

https://support.microsoft.com/en-us/help/12413/microsoft-account-what-is-family-group

the reason I say that you should create a new account for your son is that in a few short years he will want to transfer his Xbox profile to his own email and if you use Microsoft email accounts it is impossible to transfer an Xbox profile from one email
account to another.

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