Parent / Child user accounts
The only way to do what you're describing out-of-the-box is by adding a user field for Users. Then you can either check that field for child users or get a little creative with relatedTo to get parent users.

If you want to, for example, prevent a child user being linked to several parents or you want the same field to display the parent (if any), you're looking at a custom field type implemented by a plugin.