I agree with Oron that a good investigator should check all possibilities, and that it is possible that Madeleine Mccann's parents are involved in the disappearance.  I also agree that we'd need to know all the details for a better answer.

My gut feeling, however, is that they didn't do it.  I think that the parents feel guilty for having left Madeleine alone.  But it doesn't make sense to me that Kate and Gerry McCann would have engaged in a publicity campaign to find their daughter if they were the ones who had caused her disappearance.