I guess the test is to give her the same meal at home when she has no unusual activities and see if it still makes her sleepy - LOL! I'm sure she'd love that.
I'd enjoy the Cadbury factory too - I love that chocolate.
As I think about my post to you it was me who originally suggested looking at the Glycemic Index - and I guess you can tell I am ambivalent about it from my own experience. Something like popcorn - which has a moderately high GI seems to affect William's blood sugar
less than Frosted Flakes - which is much lower GI. I think the fiber in the popcorn makes a difference for him. Now maybe that isn't the case for others. I know a lot of parents with type 1 kids have a great deal of trouble with pasta making their kids blood sugar shoot up quickly. William doesn't have that. The more fat, protein and fiber the item has the less it makes his blood sugar spike - compared to just eating straight sugary items. Hope that makes sense. So ice cream has a much slower effect than an ice pop.
However the ice cream can have just as much or more carbs than something else. Maybe your child gets tired from the delayed effect of some slower acting items like ice cream and chocolate.
If your child had diabetes it would be type 1 (because she is just 5) - and tiredness can be a symptom but it is usually tiredness ALL the time - not just after eating certain things. Also there is a dramatic increase in the amount of drinking - they are just thirsty beyond belief (5 cups of water at a time type thirsty) and going to the bathroom to pee as well. Here is a list of symptoms:
http://www.childrenwithdiabetes.com/clinic/signs.htm
From what you've said I don't think whatever is affecting her is type 1 diabetes. But it could be fluctuations in blood sugar if she isn't used to the sweet stuff.
At WDW my kids would be exhausted in the afternoon/evening no matter what they are eating - especially if the weather is hot. Maybe plan to have your sweet treat in the afternoon and go back to the hotel for a nap so everyone is rested and ready to go out again in the evening for parades, fireworks, etc.
Have fun!
