I’ve been to a few. No, the whole audience does not consist of VIPs and celebrities, but you can’t buy the tickets. They’re handed out to cast and above-the-line crew, with the higher-ups given many tickets to hand out to friends, family, agents, publicists, etc. I’m pretty sure there were children at some of the R-rated premieres I’ve been to.
What else is unique about premieres? Well, there’s usually a party afterwards, with lots of good free food and drink!