I only ever read Game.
Revealing the stakes at the beginning would be asinine. Since this is Hollywood, I expect it to occur and for Ender to be played by Perez Hilton.
Despite the temporary idiocy of Rosie or Janine as Graff, the way Card explained that decision actually
makes sense. In the written word, the twist that the fleet simulation "games" were the actual war is hard hitting (although everyone really ought to have guessed, considering how many pages in the book were left, if not just through sheer intuition/deduction). In visual media, where you don't have all that "gravitas" that third person omniscient viewpoint gives you of everyone's hopes, fears, motivations etc. piling on to add to the tension, it's not, it just seems like a cheap contrived McGuffin, it would seem cheap as compared to other great movie twists, like Bruce Willis being dead in "Sixth Sense".
So instead, give it away to the audience so you're not insulted, but instead it adds to the dramatic tension, because Ender doesn't know. Nor do the other kids from the Battle School.
So that I can support, if it's done right.