Hello @firefox and welcome to Trend Micro Community!
If you are using the Trend Micro Home Network Security, you may add the Youtube website to the Denied List of the app, To do that, please refer to the steps below:
1. On your mobile device, open Home Network Security app. 2. Go to Family, then select the family member's profile. 3. Go to Settings, then tap Website Filtering. 4. On the Website Filtering page, tap Set Exceptions. 5. Tap Denied List. 6. Click Add website, then type in the website URL.
But if you are only using the Trend Micro Security program for your child's device, you can refer to the link below: