I like Swag1959's theory about Ron coming home from work early, finding the children alone, gets upset, and harms Haleigh.
In Swag1959's theory, Ron has already hidden the body before Misty returns home (I believe). So how about another twist to Swag1959's basic theory...
Ron comes home, finds Misty gone, harms Haleigh, waits for Misty to return, and then tells her while she was gone, someone harmed (can't bear to print the word) Haleigh, and now Misty needs to help Ron hide the body. That way he has something on her. "Look what happened when you were gone?"
Actually, deep down, I think Ron was responsible, Misty was there when it happened, Misty helped cover it up, and then Ron hatched the abduction plan.
I think this was done while Ron was claiming to be at work. They just look too too in cahoots.....