Well, I think it depends on the kid. I know my kids well enough to know what things might bother/disturb them and what won't. Language, for example, doesn't really bother my kids. Not that I want them saying or hearing the F-word all the time for example, but my husband has a sailor's mouth LOL, so they have heard stuff. The violence & scary stuff & overall "darkness" might be disturbing though. For example, they've never seen Hush. Some of Angel's actions on ATS would be difficult for a child/preteen to understand (sex with Darla, killing the soldier-guy in Conviction, killing Drogyn etc). Even my 10-year old doesn't quite "get" that Angel lost his soul because he had sex with Buffy, I've just framed it so far that he falls in love with Buffy & loses his soul because the curse has the "perfect happiness" clause.
As far as Twillow, they've seen OMWF many, many times, the 10 year old understands that they're "gay" but probably not more than that. Both of them think any type of kissing is "gross" at this point (although the 10-year old might be covering up his curiosity LOL, he's not really open in talking about girls, etc).