The movie City of Ember is based on a book with the same name written by Jeanne DuPrau. Overall the movie is pretty good, but just don't watch it with high expectations. It is a good movie for family entertainment and kids should love it (or at least not be board with it). It is clean, which is always a plus, and for those ideologues who worry about how Hollywood is "ramming their morals/politics/fashion/junk down our throats" this movie does none of that. Just don't expect something exceptionally complex that will make you think about your "status in life"/"state of being"/future/humanity/"the categorical imperative". So if you want entertainment, and something that will keep the kids glued to the TV for all of 90 minutes, this is it.
My advice is to just watch the movie and not think too hard about practical things like, plot, and, "Why in the world would ANYONE do something so monumentally stupid?" and "How in the world did they get there?" For example, there is a part in the movie where the main characters take a little boat ride down a gentle (if a 50% grade is gentle) underground river (think roller coaster). After going down for what seems like an eternity (to us) they somehow magically arrive at the surface about a mile above where they started out. I'm still trying to figure out how you can go "down" that far and end up above where you started, but that is merely a minor (critical) plot point.
The entire premise of the movie hinges on the idea that in order to preserve mankind "the builders" built the city of Ember below ground (the surface was uninhabitable for unspecified reasons). And for some strange reason they only gave the people enough resources, and power to last 200 years, and they even specifically designed the city to break after 200 years, with absolutely no way out except for the secret, wet and wild, boat ride through the earth that will magically bring you back up to the surface. And on top of that they specifically did not tell people that there was a way out, or even that there was something other than the city ("There is nothing beyond the darkness."). Yet to prevent "The last hope for humanity" from dying out underground they gave the first mayor a box, containing instructions on how to get out, that would automatically open in 200 years. Oh and they didn't tell the mayor what was in the box, just that they had to pass it along to the next mayor. And they had no other "back-up plan" or failsafe, just in case something normal happened, such as the mayor dying before the next mayor can receive the box (which happened). So the only instructions for how to get out of the city are sealed in a box that only one person knows about, and they don't even know what is in the box or even that they can (or should) leave the city. For the first few minutes of the movie I kept thinking, "Couldn't they have come up with a better system? One that was not dependent on a single set of instructions and ONE key (made out of two pieces of glass!). Couldn't they at least put the instruction on something more durable like metal or at least plastic, and not paper so that the main character's little sister doesn't eat them (which happens)? Wasn't anyone thinking?!?"
As soon as I stopped ignoring these obvious questions and just focused on watching the very amenable characters, and just went along with the story, the movie was much better.