If it is just YouTube you are concerned about then why not block it by creating a custom route in your router.
A nslookup for YouTube resolves to 172.217.18.238
Something like a route for 172.217.18.0 with a mask of 255.255.255.0 to a unused gateway address of something like 192.168.0.253 (depending of you subnet address.
I can drop YouTube packets originating from the Humax using the firewall. If I want to watch YouTube videos on the TV screen, I can use the app installed on the TV (which does let you parental lock) or cast them from phone.
I emailed Humax support and they said they’d pass it on as a feature request, but unlikely it would be implemented anytime soon.