If you as a parent want to change your login details, go to "Settings" → "About me". There you can change your name, your e-mail and your telephone number.
If you are a parent and want to change the child's login details, go to "Settings" → "My children" → (The child you want to change) → "General settings"
If you are logged in to a child account, press the wrench in the top right corner of the main menu. Then tap on "About Me".