I've only seen the first one, and I would let my oldest watch it - she'll be 8 in the summer. I didn't think it was bad at all. She hasn't seen it though, as she hasn't heard of Twilight so she's never asked. It's an 11 rating over here, which means you are "allowed" to watch from 8 years old if you have your parents or an adult with you.
I think it totally depends on the kid though, Kaja doesn't scare easily, but other kids might get upset and have trouble sleeping - and then I wouldn't let them watch it. As for the romance/kissing/whatever, I don't really have a problem with that. It seems that in the US (no offence) a lot of people are really uptight about that kind of thing, whereas violence is "fine". I would be more worried about my kids seeing violence than seeing some heavy kissing and a bit of flesh
That reminds me - I really need to get around to watching New Moon
