Nim's Island is a sweet little picture with much in it to enjoy, although one's suspension of disbelief gets rather challenged by having not one but three Lassies: a sea lion, an albatross, and a lizard, any one of which would have beat the original Lassie back to Mom with the message that Timmy was in the Well. But I am getting ahead of myself.
The premise is that an eleven year old girl, Nim, lives with her scientist father on a deserted island in the South Pacific. She is a big reader and a fan of the Alex Rover books (he being an Indiana Jones type adventurer). She has developed an email relationship with the author (Jody Foster), who, despite writing all these great adventure stories, is actually an agoraphobic who never leaves her apartment. Nim's father gets lost at sea, and so she writes Alex Rover to come and save her; but she gets Alexandra Rover (the writer), who forces herself to travel to the Pacific to help the little girl, though by the time she gets there she is a basket case due to her agoraphobia. So now we have these two on the island trying to deal with each other as well as with the crisis.
The contrast between Rover's public persona and the reality is absolutely hilarious, and the way she eventually overcomes her phobias, grows up, and has a real Alex Rover adventure due to coming to love Nim more than she dreads her fears, is genuinely heartwarming. Christians can understand this movie as a parable illustrating the biblical verse that says perfect love casts out fear (1 John 4:18). I know of no reason to believe that this was the film's conscious intent, but the truth is illustrated powerfully for those with biblical ears to hear, and without any morally objectionable sidetracks. Meanwhile, Nim's Island has the added bonus of giving a positive portrait of homeschooling and spinning intellectual life in general and reading in particular as "cool" for kids.
Unfortunately, there is too much of the unbelievable critter-to-the-rescue motif, which is so incredibly corny (especially to anyone who remembers the TV "Lassie" series) that it gets in the way of enjoying the good stuff. Nevertheless, I enjoyed the film overall and thought that the good elements in its plot outweighed the bad. Even that stricture would only apply to adults; undoubtedly it won't be a problem for your children. Take them to see Nim or rent it for them, and if you don't mind rolling your eyes at a scene or two, you will enjoy it too.