I live in Orlando and have spent a lot of time in the parks. If your here in the summer you will not need anything long sleaved at night (it's usually still in the 80s or high 70s even at midnight). I'm not sure about the flip flops because you're going to be walking something like 4 or 5 miles a day but if your use to wearing only flips you might be fine. People only seem to dress up at the parks if they are going to one of the nicer restaurants or plan on hitting the clubs after they leave the parks (which at 14 I imagine is something you're not doing).
you hit the hail on the head with the sunscreen cuz you're definitnely gonna need it!
personally i would NOT wear short shorts. there are TONS of people and you wouldn't want them to view you as being a hoochie mama. wear normal lenghth shorts (by normal i mean at least somewhere in the mid thigh region). i personally love bermuda shorts especially when you are sitting on rides cuz your legs won't stick to the seat.
tank tops are fine and i'd also recommend some fitted graphic t-shirts. this allows you to be cute and comfy at the same time with out looking like you are trying.
and i DO NOT recommend flip flops! i'd only wear them around the hotel. you may think they are oh so comfy for every day wear but you are totally gonna regret it when you are walking/standing in line every day all day usually for 8-12 hours a day. if you don't own sneakers then go buy some sneakers. i've actually found that they don't necessarily have to be expensive sneakers to look cute and be supportive. i love getting my sneakers from payless. sure they only last for 4-6 months at a time but they are comfy for me and pretty cheap. ($20-$25). AT LEAST buy a pair and break them in and bring them with you so that if you do wear flip flops and regret it you can switch to the sneakers.
and idk why the heck you would dress up for the evening at the parks. that really doesn't make sense. the only reason i would recommend dressing up a little is if you are going to eat somewhere nice. if you dress up and walk around in the parks you are gonna look WAY too overdone.
also in the summer it rains nearly every afternoon so i would actually recommend bringing 2 pairs of sneakers so that when one pair gets wet you have another pair to wear while those ones dry out in the hotel room. also bring some extra pairs of socks.
and bring or buy a rain poncho. you can get one from walmart for about $4 or from the parks for about $8. a poncho is much better than an umbrella cuz its hands free and covers you from your head to your lower legs so for the most part you stay nice and dry. also if you bring a backpack/purse you can always wear it with the poncho on and it won't get wet.
also you most likely will not need it since during the summer the daytime high is about 91 with many days getting even hotter than that and the night time lows only going down to about 72 but i would bring a light jacket or hoodie just in case you get chilly in air conditioned buildings.
Your clothing choices sound great, but I don't think they let you on some of the rides with flip flops on. Oh yea... and it will get cold at night so make sure you bring a jacket or something.
Hope you have fun!
I am also 14 years old and have been to Disney world a good number of times. The day outfit sounds like something that i would wear while at Disney. The night outfit also sounds amazing but i would recommend bringing a sweater or sweatshirt or something to wear over your dress just in case you get a little bit chilly. i wish you well and hope you have a wonderful first trip to Disney.
p.s. I would also recommend wearing a swim suit underneath your day outfit just in case you go on any water rides because i know i do. have a wonderful time on your first trip.