We took our 8 yr old daughter last October. It was VERY hot still. The ice and backpack are a definent must.
Something we noticed was that almost all the rides had nets and such for bags and things. This was a great thing for keeping the bag handy.
You'll want to bring the insulin because we experienced just what some others here have mentioned. Our daughter was so excited that she went up considerably at times.
Security will want to look in all the bags but they never even hinted at questioning the supplies, though they did want to look in all the bags. We prepped by having them all unzipped before we went through. They didn't question the snacks either. They were great!
Have a BALL and don't forget to check out the child/parent swap thing Disney does. An adult can wait in line then switch out for a child at the end of the line.
We didn't know about this until we got back but this would have been a great thing.
Have a SUPER trip and let us all know how it goes!