Selecting Lock Apps & Games should give you a list of Apps which can be Blocked, Locked or Unlocked: Hopefully YouTube will be shown there and can be configured as you wish.
Under the YouTube link:
When you open YouTube for the first time the Restricted Mode will be set to Enabled. This will hide content you might prefer not to be shown during a search.
To reset this:
Select Settings from the menu
Select Restricted Mode
It's not clear what happens if you have previously opened YouTube and wish to make subsequent changes.
I find it hard to believe that VM would not provide adequate Parental Controls, on equipment provided by themselves and which is being installed in family households. It's probably just a case of finding out where these controls are.