Adding a child account is currently unsupported in our app. We’re sorry for the inconvenience and may consider adding support for child accounts in the future.
In the meantime, you can remove the child account from the Family Safety section of the parent Outlook.com account.
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Sign in to https://familysafety.microsoft.com using the parent account.
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From the Select a child to view or edit their settings section, select Remove.
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Select the corresponding child account and select Remove.
Once completed, try adding the account again.