Child's account as default login user
Hide a User Account in macOS
This works in OS X Yosemite (10.10 and newer). The general syntax to use to hide an account is as follows, replacing admin with the user home directory of the account to no longer display:
Hiding from User Menu
For example, to hide the user account “admin” on a Mac with the given user directory being /Users/admin, the syntax would be:
$ sudo dscl . create /Users/admin IsHidden 1

then reboot for the change to take effect.
Hiding in Finder
You can move the hidden user's home directory to a place that's not visible from the Finder. And you can remove the hidden user's Public Folder share point.
This command moves the home directory of &quot;admin&quot; to /var, a hidden directory:
$ sudo mv /Users/admin /var/admin

This command updates the user record of &quot;hiddenuser&quot; with the new home directory path in /var:
$ sudo dscl . create /Users/admin NFSHomeDirectory /var/admin

This command removes the Public Folder share point for the user with the long name &quot;Hidden User”:
$ sudo dscl . delete &quot;/SharePoints/Admin's Public Folder&quot;

Unhiding
$ sudo dscl . create /Users/admin IsHidden 0

Changing the number from 1 to 0 changes the hidden status of the account. You must reboot for changes to take effect.

Source:
https://support.apple.com/en-us/HT203998