Nevermind that the movie and the poster are meant for young kids. The marketing geniuses at whatever studio this is recognize that part of being "cool" today is being downright rude, disrespectful, combative and a la reality show- vulgar!
There's primarily two words that come to mind as actually belonging in the sentence. Well, maybe three- but given the mouths on some of today's kids, "heck" is probably not one of them. So, we're back to two possibilities. The first one, comparitively tame, is hell; Where the hell are we is what the smurfs may be thinking or saying. Unfortunately, the more likely word that will be substituted by kids, of all ages, is a four letter words that goes something like this: F***!
This is a kids movie. It's meant to be watched by kids. The people behind it however, are adults. They know exactly what they're doing and they chose to go with the "where the #*@! are we" approach. You might even see some kids reading the poster outside the movie theaters outloud. If they happen to use the words those who designed the poster are banking on, and you tell them to watch their language, just don't be too surprised if they come back with "I've got your language right here"! Afterall, that was another genius slogan by Burger King not long ago, you might remember it: "I've got your Whopper right here"!