OP, I think it is a wonderful way to celebrate a sweet moment of accomplishment. You could get a celebration button, or order a cake for one of your TS meals that she wears the cap and gown to...I would love to see an adorable little 5/6 year old in a cap and gown celebrating their first academic milestone of many in Disney World! It would totally make me smile and make my day...I will never forget my oldest's last day of Kindergarten - I think I cried more on that day than his first day of K! And now we are in the midst of preparing to watch him graduate 8th grade - and yes, apparently in the greater Chicago area, 8th grade graduation is a HUGE deal - caps, gowns, 8th grade pictures, dinner dances, ceremonies, the works. Seems odd to me since we have only lived in IL for 4 years and did not do this for 8th graders in Southeastern/Mid-Michigan where we are originally from, but hey....I will always take any opportunity to celebrate and honor another step that my kids have completed in the academic world.
And don't worry about the cranksters here...some of these very same posters criticized me yesterday for surprising my kids with a trip in November that my kids will remember the Rest Of Their Lives and still talk about all the time...we had one little issue in a well-planned surprise and the issue worked itself out very nicely for all involved. And the criticisms came SIX MONTHS after the trip!!! LOL Seriously...some people cannot help raining on other people's parades. It's sad, really....but just ignore them and celebrate your adorable little ones Kindergarten graduation however you want!!!
ETA: Since you are going to Akershus, you have a few minutes to talk to Belle when you are getting your family picture taken with her. For our family of 5, they took two pics - one with the kids only and one with all 5 of us. Maybe if your DD wears her cap and gown to Akershus, you can get one pic of all the girls or your whole family, and one just of Belle and your little graduate? That would be adorable - and what a memory keepsake of her fave princess! If she wanted to wear a princess dress to the dinner, she could just wear it under her gown, and take off the gown before dinner starts. (We loved Akershus...Cinderella flirted with my 13 year DS, and he flirted back and asked her what she was doing at Midnight LOL She almost broke character, but quickly recovered and told him she would be back in the castle for the night and he should be too, so his mother didn't worry about him! LOL It was sooo funny! And the food was really good, too!)
Have fun!!!