Set up each phone deliberately
On iPhone, limit adult websites in Screen Time
Open Settings, choose Screen Time, turn on Content & Privacy Restrictions, then open App Store, Media, Web & Games and Web Content. Choose Limit Adult Websites. Add known problem destinations under Never Allow rather than assuming the automatic list catches every site.
On Android, turn on web content filters
On supported Android versions, open Settings, Parental controls, and Web content filters. Turn on Block explicit sites and Filter explicit search results. For a supervised child account, Family Link can also restrict Chrome to approved sites or attempt to block explicit sites.
Close the obvious alternate routes
Review other browsers, social apps, private browsing, app-store installs, and cellular-data use. Keep only the apps you need, use each app's sensitive-content controls, and remove an alternate browser if it exists mainly as a workaround.
Put the change credential outside the impulse
For a self-lockdown, ask a trusted adult to hold the Screen Time or parental-control credential. Do not reuse the everyday phone unlock code. Write down the legitimate reasons for temporary access and require a one-hour wait for nonurgent changes.