First off, if you are a Christian, do not let your kids see this movie. The main idea of this movie is essentially that the way to get rid of an evil spirit is just to "be happy" enough, which is obviously not what the Bible teaches.
Second, even if your family isn't religious, your kids have no business watching this movie. Either they will be creeped out by it, in which case they are too young (like I said, the "G" rating is outrageous), or they will not be creeped out by it, in which case they are too old or too mature to be wasting their time on this mindless drivel.
Not only this movie, but the entire genre of "overly-happy sappy generic messages, pretty colours" is an insult to any one who has ever experienced the real world. Similar to shows and movies like My little Pony Friendship is Magic, Sofia the First, Puppy in My pocket, or Care Bears, they are overly-sweet, vapid, pandering, the characters aren't put in any danger, or the characters themselves have no personality. While I was relieved that the movie WASN'T overly-sappy, or this decade's version of "A Troll in Central Park", it instead tries to be hip, cool and relevant, all while failing. Not to mention that the characters aren't put in any danger. Don't let your kids watch Trolls, or the upcoming TV show. Buying them the toys is fine, but keep them away from the upcoming animated series. Trolls has no educational or moral value. The whole theme of the entire Troll Dolls franchise is that all your problems will just go away if only you and your friends just "Be Happy" enough. American Greetings, if you're reading this, than all I have to say to you is try telling that to the the little kid who just lost a friend to cancer, or the kid who's dog just got put to sleep.
There are some who would defend this movie as being just harmless fun even if they do acknowledge that it doesn't have a very strong moral foundation. To that I say, first of all, anyone over age 4 will be bored to tears over this intelligence-insulting movie. It is not fun, and as for me, I have no fond nostalgic memories of the Troll dolls themselves. Second, little kids need to be learning how to be effectively dealing with the problems life throws their way, and learning that "Happiness" isn't always enough. They also need to learn the harsh realities of life, like the fact that holding hands and standing in a circle with your friends shouting the "Happiness is inside you" message over and over again will not bring your dead grandmother back.