How is my child able to add an allowed website when I've restricted their web access?

HI there,

I tried restricted my son's web access on his iPad to only approved websites by going into Settings > Screen Time > Content & Privacy Restrictions > Content Restrictions > Web Content > and I added my own list of approved sites.

But somehow he was able to add websites on his own, including YouTube and Facebook and some other sites. Is there a way to block that?

TIA.