Until Apple sorts out this issue, this is a workaround which basically consists in removing Safari and having Firefox [or other browser] as a web browser on the iPad/iPhone which is considerd then as an App and can be disallowed:
Settings -> screen time -> content restrictions -> Apps -> allow all apps or 17+
Content restrictions -> Allowed apps -> Switch off Safari
In Apple store download Firefox [or other browser] and install
In Screen Time -> Always allowed -> make sure Firefox/other browser is amongst the list of applications that are not allowed during downtime
You will have to always allow apps 17+ for Firefox to remain usable, however if you have set up their iPad/iPhone so that the children must ask your approval whenever they download an application this should ensure that they do not download applications that are not suitable for them.
However, if you find any problems with these settings in that they can then access content that they should not, please let me know as I am using these at the moment with my daughter.
Thank you.
Hope this helps!
Laura