it won't be unrestrained which makes a big difference. Be ready for criticism from your friends, at least one of your kids (some will recognize your wisdom), your kid's friends and the parents of your kid's friends about how ignorant and cruel you are. And a few other dumb kids with smart phones will laugh at and shame your kids for having a flip phone, loudly bring it to everyone's attention, and all of the others around will laugh with them because that is what kids do.
We made the same choice (and are still doing this with our caboose). Flip phones are a great stepping stone appropriate for young minds. Check the flip phones carefully too periodically. They still have dumbed down smart phone features using WiFi or even through a restricted data plan.
We encourage them to get smart phones once they graduate from high school and even pay for them. We didn't even do flip phones until they started driving. Of our older that have graduated, none of them have criticized our decision to not allow smart phones before or during high school. And they want to make sure the younger ones get the same treatment.