Even though "Earth" is a G-rated Disney nature movie, you may not want to take small children to see it. The death scenes of some of the animals would be too traumatic for them. Heck, they were traumatic for me! The most dramatic sequence is of a polar bear dying of starvation and exhaustion.
"Earth," although beautiful, is a bit disjointed. The main subject--the migratory habits of the planet's animals--is too big a subject to cover in a 90-minute movie. So the film skips around a lot and just mentions just a few of them. Another reason not to take small children--they'd have a hard time understanding why the film keeps going back and forth between subjects.
On the other hand, you must stay for the closing credits! They show how the cinematographers made the movie--not overhead in an airplane, but in a hot air balloon. And the final logo is dramatic--CGI shows Cinderella's castle completely covered in snow and ice.
One piece of trivia that I learned--there are 42 types of birds of paradise that live in New Guinea.