I was reading and ready to respond until I came to your post. You said what I did. Kids can make reasonable choices at certain ages. My kids had phones at age 10. Still in elementary school, but after school activities, etc. It was imperative to be able to connect at times, especially as their world widened as they got older. At younger age, tracking a must. As they got older, knowledge and agreements on both sides. Talking to your kids and having a relationship of trust works with many. Some kids have different personalities and may want to be more private - somehow those are the ones I'd worry about. (I had one.) Other kids were more open.
Anyway trust comes from actions on both sides. Conversation is imperative and a child should be aware that you are looking out for them. If they don't agree, maybe they don't need the phone.