Cadet Portal and MyCadet are separate applications.
Cadet Portal is used by cadets and adult volunteers, and MyCadet is used by parents and guardians.
Cadets have a Cadet Portal username name and password, which is used to log into their Cadet Portal account.
The Cadet Portal log in page looks like this:
Their parent/ guardian has a separate username and password for MyCadet, which they can use to log into their MyCadet account.
Their MyCadet log in page looks like this:

Please note: Parents/ guardians are able to access their child's Cadet Portal account via their MyCadet account, by clicking 'Check Progress'.

This takes them out of MyCadet and into Cadet Portal.
If parents/ guardians need help registering or logging into MyCadet, they should visit the guidance here.
All screenshots and references to persons are taken from the Westminster Training or Development environments. All information contained in these guides has been anonymised and no real personal information is displayed.