Our oldest(12) has my wife's old iphone. Its $20 to add her to our sprint plan and she has unlimited everything.
She uses it mostly for games and movie making, then for texts, then for internet, and phone calls is a distant last place.
Just mentioning this because if the difference is $10 between a bare bones text n talk plan and having your kid as an added line to your plan, it may be worth the difference.
...at the same time, mine doesnt play video games other than the few apps on her phone, so we thought it was worth it to essentially combine cell and games. If she were a huge gamer and that cost us a bunch too, I may have had reservations.
Limiting screen time and app time is really easy. There are phone settings to restrict use, app settings to restrict use, and rules arpund use too. Foolproof so far.