Diabetes type 1 and 2 are reversible.
Type 2 is much easier than type 1 to get rid of.
I do not want to scare you, but I think you need do some research to find out how many days after stopping the use of insulin you can have your child fast.
I would suggest you watch some videos on YouTube by Dr. Fung. He makes all his patients fast intermittently. Also there are other parents like you who talk about their experiences.
I love the idea that you parents are fasting with your child. You will learn with your child what your body goes through.
Fasting is amazing, cure for all, but for type 1 diabetic you have to be cautious.
Fasting with water means just that. No salt, or supplements, body will produce what it needs. The things you need to do is to rest, drink water (filtered and warm) by thirst and stay warm, this affords your body to use all its energy in healing. You all will experience detox symptoms, depending on what your eating history is, headaches, pain in joints, nausea, acid reflux, dizziness are common symptoms. You will not face all symptoms at the same time.
Most importantly after fasting a change in eating lifestyle will need to change. You don't say what is your diet, but I would suggest that eat nothing but vegetables, no fuits obviously, no meats, milk, cheese, fish or processed foods. Make yourself and let your child help you make salads and some cooked meals with vegetables, avoid carrots and potatoes.
Read books by Dr. Neal Barnard and Dr. Gabriel Cousens. They talk about why we need to eat a raw vegan diet. And search my messages for to find a PDF link to Dr. Shelton's book on fasting. What a wealth of knowledge!
It would be interesting to know what is your child's blood glucose. Also what is blood Sugar before and after fast each day. The