I'm not sure what kind of dress that is - what kind of material is it?
When we go in August - we wear shorts (and NOT denim shorts - but light, sports material shorts) and light colored, light weight t-shirts (we don't do tanks - because we burn and less skin exposure the better). But I specifically buy either tru-dri material (like for sports) or those really cheap feeling t-shirts that seem like they'll come apart after 5 washings because they are so thin. And stay away from dark colors - white or mostly white is the way to go.
That said, my dd has worn dresses before and honestly with that 'breeze' that she gets from top to bottom I think it's been a bit cooler for her. But it was cotton only and light and very, very comfortable - and it was a spaghetti strap dress that time (lots of sunscreen, but worked out ok).
Sorry - can't help you about the rides - I would say it would depend on the length whether or not it would get in the way at all. But there are a lot of people of different cultures who wear skirts only - so I'm sure that many people have done it without too much of an issue.