I think B said that if he told a secret, it would no longer be a secret.
Certainly kids keep secrets, as evidenced through child sexual abuse. The perp will most always play the secret game with the child, unless he/she is a very dumb perp, or fairly mentally challenged. But...in my experience with interviewing kids, they eventually begin to trip themselves up, even if they are fearful of the perp. Once they see the interviewer as someone to be trusted, often times they will spill the beans w/out much skill on the interviewer's part. It's just so much different than interviewing adults. (Plus, there is that fine line there that the interviewer not present leading questions; if so, the child most often will follow that lead and then you get into fabrication.) So, to kind of answer your question, Camper, sometimes it does matter how big a secret, and how much value the family puts on it. But, being interrogated by LE, and by either a social worker, psychologist or psychiatrist (I can't remember which now) with LE observing, I have to think that some clues could have been given by B that would have indicated some thing that went on in the house that night. Of course what we don't have, is the interview to read....to actually read the questions that were asked of B. So much lies in the interviewer and his/her techniques.
That's why if BDI, I think he truly doesn't know it...that the parents took over, made him think she was okay, sent him to bed, and then staged it for his protection. The only real way that I can go along with this train of thought, however, is if I follow BlueCrab's theory that the GJ figured it all out while B was on the stand. I just don't believe that as a 10 year old, he could withstand interrogation and not give something away.