From the start, I believed they were such total creeps they wouldn't know fact from fiction. They seem like exactly the kind of sleazy people who would sell or rape or kill their own kid, but I'm going to withhold judgement for awhile. Even sleazy people's children can be kidnapped unbeknownst to them. They might be at fault because they're neglectful, but not necessarily part of some plot. (Hooooo boy, am I stretching to be "fair.") The truth is out there.
POSSIBLE AND MORE LIKELY
1. Relative or friend of the family who is known by the whole family has had his eye on the little girl for awhile and knows when the parents' attention is diverted (but isn't involved in swinging) and takes the opportunity to break in and abduct the girl.
2. The little girl caught the eye of the neighbor when she went to sell Girl Scout cookies. He's got child porn in his house, but he's never done anything to hurt children before. He learns of the parents' lifestyle (which he himself hasn't done before) and mistakenly thinks the parents won't be so "judgemental" about his sexual fantasies. He runs into the mother on her "girl's night out", maybe gets propositioned by the mother to join in one of their parties, so he knows what is going on in the garage that night, and he likely knows that their alarm doesn't work properly and he takes advantage of both situations and breaks in and abducts girl, she dies "accidentally" and he buries her out in the desert.
ROFLMAO! What are you going to say about them once you do make up your mind? Aren't you afraid FR's bible-thumper villagers aren't going to let you set at the lunch table with the "cool kids" for taking such a radical stand?