yes you can order kids meals for adults
It's good to know that she'll be able to get kids meal, but also don't underestimate the huge amount of calories you'll both burn in Florida.
the amount of food someone can eat can be different at home, and in Florida after over 10 miles of walking every day.
My point is that it's often better to leave a bite or two because you're stuffed, than ordering something smaller and going hungry all day long.
To echo what a few PPs said, it will probably be best to order an adult CS without the sides, than a full kids meal.
Now you didn't talk about YOU.
The excess food on your girlfriend's plate might be what lacks on your own plate.
Then finally, if you get an adult CS inside the parks, some come with a bag of chips. If you don't eat it during the meal, it can come in handy (for you or your GF) during the day as a quick snack
You'll be amazed at how much and how often guests can eat while in WDW
whatever you choose to do : listen to your tummy
