That would be perfectly fine and I can almost promise you that, with khakis and a polo, they will be some of the nicest dressed kids in there (and good for you/them for caring how they look...the world has forgotten how to look nice at nice places, or so it seems). Even so, although a "signature" restaurant, they realize that people will be coming directly from a theme park, so the expectations are managed accordingly. Last time we went, we rented a locker at the MK to keep some not-so-sweaty clothes available for dinner there, but that was probably overkill. Pretty sure that the only restaurant that truly enforces a very strict dress code would be V&A. As long as you're steering clear of cut-off jeans and t-shirts, you should be fine.