An administrator user can allow the managed user to use Dictation with these steps:

1. If logged in as the managed user and System Preferences is open, quit System Preferences.
2. Choose Apple menu () > System Preferences.
3. Click Parental Controls.
4. Click the lock icon in the lower-left corner of the window.
5. Enter an administrator user's name and password, then click Unlock.
6. Select the name of the managed user account.
7. Click the Other tab.
8. Uncheck "Disable use of Dication".