I spent some time learning about the family sharing and child restrictions available on the iPhones. I've been pleased with it. She only has the bare minimum apps and a couple educational ones (no social media yet, restricted web access), and is not able to add any without authorization by a parent on the account. We plan on allowing her to earn entertainment/game apps with good grades, etc. She reads like crazy, so we don't restrict access to books borrowed for free through a library app.
She is actually without it this week, as a result of discipline (nothing terrible, but an excellent bargaining chip we didn't have before). Great responsibly comes with bigger consequences.
I am not sure how we will handle instances of loss or damage, which are sure to occur. May have to contribute monetarily, work it off, be without it for a while, etc.
As other have mentioned, it is a fact of life now. It is important that they adopt the technology that is pervasive in our society. They need to be comfortable with it and have a healthy relationship with it. It is a large part of socializing now, which, again, is important to allow in a healthy, responsible way. I would rather be an early, involved part of that to help steer it when it inevitably goes slightly off course.
Edit: To add, I believe that as parents, our best teaching tool will be the way our kids view us using our phones. When, for what and how often. And I suck at it. I am way too prone to pulling out my phone and flipping through a feed at any idle moment. I know I need to set a better example. But we do have strict times, such as dinner or in certain social settings, where we don't. We need more of those times, and it is always a rewarding feeling when we do an activity as a family where I realize I didn't bother or even think to glance at my phone for hours. It is good to seek out more of those experiences, and our kids notice it.