What it teaches
Logging in, which every child is asked to do in their first week at school and which nobody teaches. The dots are the whole point: a password cannot be checked by reading back what you typed, so it has to be typed carefully the first time.
It also teaches that a username and a password are two different things with two different jobs — which is not obvious when both are just words in boxes.
How it works
A card shows the username and the password. The child types the username, and the game moves to the password box on its own. That box shows dots as it fills.
A wrong key is refused, and the message says to check the card rather than the box, because the box cannot help.
What to look for
Nothing here is stored or sent. The usernames and passwords are made up and go nowhere, and no box on this site ever asks a child for a real password.
This is a good moment to say out loud that a real password is never told to anyone — not a friend, and not a website that asks for it twice.