What age are your children?
I haven't seen Gnomeo and Juliet, and while I doubt it ends in Gnomeo, Juliet, and Gnomeo's mom all dieing (like in the source material), I would suspect there is something sad.
As for Rango, while I don't really recall anything sad, I found it a little inappropriate for really little kids. I mean, you've got a giant snake that says he came from Hell, and a Armadillo squished in half by a tire (but still alive and talking). While I have a warped sense of humor, and was OK with my 10 and 14 year old watching it, if I had seen it with them only first, I would have avoided taking my 3 and 5 year old nieces to the movie. They didn't cry or anything, but there were a few points where I was thinking "wow, I'm not sure about this...". Rango was REALLY FUNNY though. I enjoyed it a lot.