Child's account as default login user I'm a complete newbie at macOS. 

Bought a new MacBook Air for my eight year old child as that is what the school specified. Followed the online instructions for setting up family sharing then created a child's account. However when I boot it up the initial login screen only shows my own account and not my kid's one.

If I logout then both our accounts are shown on the login screen, but as it's my young 'un's laptop I'd like for his picture to come up at the initial login screen, and preferably be the only account showing there.

I have done some searching and found the Default login drop down in the Users and Settings page but it's greyed out at the moment.

I wondered if it could be to do with the fact that when you set up the account it gives it a user@icloud.com address, so is not a local account. But I can't figure out how to make it a local account, or if that is even part of the issue?
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