The high level steps at the following:

1. Create a new Google account for this tablet

2. Remove or disable not in use apps

3. Setup Parental Controls on Google Play (https://support.google.com/googleplay/answer/1075738?hl=en)

4. Creating and Using Restricted Profiles to control which apps able to run under the kids account (https://www.wikihow.tech/Set-Up-Parental-Controls-in-Android)

5. Enable Restricted Mode on YouTube Apps (https://support.google.com/youtube/answer/174084?co=GENIE.Platform%3DAndroid&hl=en)

(I didn't sign-in google account, it also able to enable restricted mode on the apps)

Reference:

https://www.techadvisor.co.uk/how-to/google-android/how-use-android-parental-controls-3461359/

http://tabletmonkeys.com/onda-v80-8-inch-android-7-0-tablet-for-73-launch-january-7/

https://kknews.cc/zh-hk/digital/y2yrjlk.html

1. You need to login YouTube by using your Google account

2. Click on your account

3. Click "Setting"

4. Click "General"

5. Enable "Restricted Mode"

Done

You will see those video being block:

Reference:

Enable "Restricted mode" on YouTube

https://support.google.com/youtube/answer/174084?co=GENIE.Platform%3DDesktop&hl=zh-Hant

「Elsagate」變成家長惡夢！　這APP讓孩安心看Youtube