Users without admin privileges should only be able to access the Today, Learning and Resource pages. If they are entering the Admin page, that must mean that they are logging in as you.
Unfortunately, we don't have a password system to prevent this from happening. We considered it, but decided against it because of the potential to really bog down performance.
Did you add user icons/photos to the login screen? This might help to ensure that your children only log in under their own icon. If you haven't found that feature yet, it is called "Set icon," available under the top right triangle on each user's page in the Admin section.