Safari restrictions won't filter a simple safari search for "****"

I'm trying to set up my son's iPhone 6SE to filter everything. I've turned on parental restrictions, set it to "Limit adult content", and added a ton of specific sites to the don't allow list. Those sites, and similar ones, are blocked fine. However, a simple Safari search of something as blatant as "****" will bring up every image imaginable. I've changed every setting I can find, but short of disabling safari entirely (not an option) I can't get the safari search to filter. Can someone enlighten me on whether this is operator error (most likely) or a larger problem? Thank you!