I certainly don't think they murdered their own child, that's frankly ridiculous, however I, like everyone else, have no idea what actually happened to the poor little thing. I think it's possible that she was kidnapped and then abused and murdered. I also think it's possible that she was kidnapped and is alive somewhere. Thirdly there is the possibility that she died accidentally perhaps because she had medication administered to her to make her sleep, and her parents panicked, in which case they are in hell anyway. I think the latter is more unlikely though because once they had 'got away with it' so to speak, it would be counter productive to keep the story as alive as they are keen to keep it.
I'm really not sure why people are so emphatic that they 'know' what happened, when clearly no-one, or at least not many people, do.
I still live in hope that she is alive, as apparently do her parents.