Yes, this movie is R-rated.
However, the messages are very good, and it teaches the meaning of friendship, trust, and appreciation of your family.
The language is strong and constant, I will say that, but if you don't like the language, watch it on TV, where they will most likely edit the strongest of words (I first saw it on TV, cause at the time, I wasn't old enough to see the uncut version).
There are some very brief sexual references, and it's not something you need to worry about, unless you're really uncomfortable with it.
As for violence, the only bad thing is that you see a dead body ONCE for only a few seconds (it's not horribly graphic, it may disturb some viewers, though).
The main characters (the kids) smoke cigarettes a lot, but it's definitely not something you should worry about (unless you don't like that sort of stuff).
Again, the messages of trust & friendship are obviously there, and it's actually rather uplifting towards the end.