My DD has Type 1 and we have been to Disney World once,
Disneyland many times, and Disney cruises 3 times since her diagnosis. Have never had a problem. She was diagnosed at 9 years of age and is now 16.
The important thing is that she test her blood more often than when she's at home. All the running around at WDW will burn sugars and she may be more apt for going low. I agree with letting her run her BGs a little higher and not correct too much. We always carry emergency snacks with us, juice boxes, PB crackers, fruit chews, glucose tabs/gummies, etc. for those pesky lows.
Take twice as many supplies as you think you will need, (test strips, syringes, insulin, extra meter, etc.) We love using the FRIO Pack in the park to keep her insulin cool.
As far as the dining plan goes, as a Type 1 she can eat whatever any other healthy 8 year old would eat, she just needs to cover her carbs with insulin. I would also recommend getting the Calorie King Book. It will help with carb counting. WDW will not provide carbs even when asked.
Type 1 is daunting at first. But, as the previous poster said, as time goes by it gets easier and just becomes "normal." Eventually getting a pump and CGM will be ideal for managing her diabetes.
Just be more vigilant and enjoy your trip.