Event Registrations will only allow 1 user submission unless specifically stated in the settings. Since the parent already registered once, it will prevent them from registering the other child.
  1. From the menu in the top left select Enrollment Management
  2. Navigate to Events > Event Registration Forms
  3. Click Edit for the Event Registration Form
  4. On the General Tab, mark the check box for: Allow Authenticated Users to Register Multiple Times
  5. Click Save