Screen Time - How do you allow specific websites for Safari?
You should be able to access Prevent web content in Screen Time. See my summary below or follow a step by step guide here.
iOS can automatically filter website content to limit access to adult content in Safari and apps on your device. You can also add specific websites to an approved or blocked list, or you can limit access to only approved websites.
Follow these steps:
- Go to Settings > Screen Time.
- Tap Content & Privacy Restrictions and enter your Screen Time passcode.
- Tap Content Restrictions, then tap Web Content.
- Choose Unrestricted Access, Limit Adult Websites or Allowed Websites Only.
Depending on the access you allow, you may need to add information, such as the website that you want to restrict.
Source
It's very complicated (deep in the settings menus).
Settings > Screen Time > Content & Privacy Restrictions > Content Restrictions > Web Content
Under ALWAYS ALLOW click Add Website then add the URL of the website you want to always allow, e.g.: "www.khanacademy.org"