My 17 year old DD has Type 1 Diabetes. We always carry all of her diabetic supplies on board with us. We have a cosmetic travel bag in which we pack her insulin, syringes, monitor, etc. We also carry with us a letter from her doctor verifying her condition and an extra set of prescriptions just in case anything should get lost or broken. We have never had a problem going through security. We simply place her bag on the x-ray machine and it goes through quite easily. We have never been stopped for not having the insulin in its original box. We usually transport it in a "cooler" type container. We do, however, carry the empty boxes with us just in case there is a problem. They can be easily folded down and kept with the prescriptions.
If your child will need an injection during the flight make sure your alert the flight attendant. In most cases, we have found them to be very helpful. Also, carry food with you. We were stuck once on the tarmac for over an hour and my DD suffered a "low". There was no food on the plane but I had brought snacks with me just in case.
Make sure that the prescriptions you bring with you are original prescriptions and not copies. Some states will not accept them unless they are the originals. Do not rely on the pharmacist calling your physician. We found out the hard way that in some states they will only give you one package (10) syringes instead of a whole box if they have to call. Plus they do charge you your full co-pay for the one package.
If you need any further info, please feel free to send me a PM. We have been travelling like this for the past 14 years and have probably experienced every problem you can think of.