I would be very suprised if the 4 year old concerned does not know who Madeline Mcann is. I have a 5 yr old who dosent exactly come home from school and turn on sky news but he knows who this little girl is because everyone has talked about her at some point in the last few months, even kids at school. So to say that this child hasnt been exposed to it seems to me to be pretty short sighted.
As far as people believing in things like this, Do you not think there is a very distinct possibility that it is believed because people want to believe it not because there is anything to back it up. After all religion as a whole is probably the biggest bluff in the history of the world, but it gives more comfort to more people than anything else.
I really believe this type of thing fits into the same catagory, i would rather believe that there is an afterlife and that certain people, children or otherwise can provide us with clues that it exists. The problem is nothing that has ever happened (exactly the same as religion) has ever even remotely given a good indication that it does exist, that couldnt be explained with a more sensible answer.