I've looked into a smart phone with the same goal, but I think by definition a smart phone will always include some way to access the internet.
That said, keep in mind, the fundamental purpose of GOS is to give the USER more control over their device and information, not to take away control/features from the user. GOS is great for someone who wants to turn it into a minimalist phone for themselves, but that's because they are in control.
Perhaps you've run into the Gabb phone- that's the closest thing I've personally found to a smart phone that can remove internet connectivity. Not sure about compatibility outside of the US though.
Also, sidenote- one parent to another, when you try to restrict/protect a child from something they want by depriving them of it, they will often simply find another means to get it behind your back and it could hurt them more than if they are taught boundaries, conduct and consequences. Easier said than accomplished, though.