Once you have joined a class, the name of your child cannot be modified. However, if you see a mistake, you can quit the class, join it again and add the correct name.
To do so, click on the little arrow next to the class name in the menu “class” and “quit the class”. Then, click on the icon to join the class, choose “parent” and enter the class key. Before selecting your child, click on “modify”, you will then be able to change the name. Confirm, select the child who’s name has been modified and then click “ok”.