Creating Xbox live account for child. Page not found problem after entering my credentials

I had exactly the same problem. I managed to find a solution by contacting support. What worked for me:

1. Log into the child's Microsoft account.

2. Go to https://social.xbox.com/changegamertag

This allowed me to give them a gamer tag which also seemed to provide/activate their Xbox account.

Hey Vincent,

Were you ever able to create Epic accounts linked to your xbox children accounts? I'm not having the exact same technical issue but I am having the exact same "support" experience you are. Lets just say its truly Epic. In my case I'm only trying link one
nameless account created under an Xbox child account to a new Epic account. I'm able to get all the way to the point where it finds my child's Xbox nameless Fortnite account but after entering all the required information and clicking create I'm simply returned
to the main Epic login screen. Entering the credentials I just created (or thought I did) return an "credentials you are using are invalid" message.

Erik

Hi, my name is Marvina and I will be happy to help you.

I'm sorry you have this problem. Follow the instructions below for a solution.

This may mean that there are problems with Xbox Live, that you entered an incorrect email address or password, or that there is a problem with your Microsoft account.
To resolve this issue, try the following:

Solutions
Check the Xbox status page. If you see any services with alerts, expand the service, scroll to Notifications, and sign in to receive a message when the service is back up and running.

Xbox status
Make sure you have entered the correct email address and password.

If Microsoft notices unusual activity in your Outlook.com or Hotmail.com account, we may temporarily block it for your protection. If we locked your account, log in to account.live.com and follow the instructions to unlock your account.

If you have more than one Microsoft account, you might be trying to sign in with the wrong account. You have to sign in with the account that is associated with Xbox Live. To make sure you're using the correct Microsoft account:
Sign in at the top of any Xbox.com page, like this one.

Use the same Microsoft account email address and password that you're trying to use on your console.
If there are any problems with your Microsoft account, you will be prompted to correct those problems.

If you're signed in to the wrong gamertag, or if you're prompted to create a new Microsoft account, you're probably using the wrong Microsoft account.

If the Xbox Live account is a child account, be sure to enter the credentials for the child's Microsoft account. You must use the same credentials that were used to create the child's account.

If you can't remember your Microsoft account password, you may need to reset your password before you can access Xbox Live.
If you're still having the issue, contact Xbox Support.

Startup Helper Application Information source (credits):
https://support.xbox.com/thread/err...8015D000,problem with your account Microsoft.

I hope I can help you solve it.

Best regards,
Marvina.

im having a problem with that, everytime i enter there it says "We are sorry, the page you requested cannot be found" and i dont know how to fix it

PS: Im NOT in a child account

Had the same problem when trying to start a child account. I used an adult account that was created on another xbox and downloaded to the xbox where the child account was. The account had to be "promoted" on the new xbox (this can be done under Settings>Account).
After this it was an option when the "parental consent required" screen came up. Chose the newly promoted parent account, followed the steps, including entering a valid credit card again, and the child account was created successfully.