* Buy one themselves
* Get / use a friend's
* Use public internet access points
* Need one for school
* Use the smart fridge / tv / gaming console / anything with a browser
* Access stuff in Minecraft, Roblox, etc
You can only stop it by going offline and raising them in a cabin in the woods, which is a whole other thing.
What you can do is give them The Talk, of course (but that only helps / prevents to a point, it's more to prepare them for what they may find or how they can identify problematic things). And the other is to push back as a community effort, with e.g. many schools banning kids from having phones in the first place.