I don’t think there are any bad answers here. Also sorry to hear the news but good on you for reaching out.
Listen. You have a bunch of great perspectives and suggestions here.
My submission is that you guys may not find an app that fits your (your kids) style. I was diagnosed nearly 20 years ago (turning 33 this year) and I still haven’t been able to find a piece of technology that I find beneficial for carb counting.
If you guys can swing it keep your kid in touch with a dietician (more so as puberty kicks in and they grow and hormones and yada yada). This will be good for them and honestly I find my portion sizing and reminder meetings can really help hone in numbers for A1C.
Other than that, your kid will grow and become more sensitive to how his/her body reacts to certain foods. It really is a learning curve, it will be for both of you.
Wish you all the best and a healthy kid!