Great question. Safari’s built-in controls don’t let you disable the Private Browsing feature itself. However, the Content & Privacy Restrictions you set in Screen Time (like “Limit Adult Websites”) still apply even in Private Browsing mode. So the filtering does work, but it’s not foolproof.
For more robust monitoring, I use Parentaler. It allows you to see their browsing history regardless of whether they use private mode, closing that loophole completely. It gives me much better peace of mind to see the full picture.