In a Paris Match interview on the 4th of September 2007 :
Mrs McCann : Peut-être parce qu’ils grandissent, ils s’expriment mieux. Quand ils voient le chat de Madeleine, ils disent “Maddie”• Quand ils voient son sac, ils disent “Maddie”.
(this is a translation and I'm retranslating) May be because (the twins) grow up, they express themselves better. When they see Madeleine's cat (CC or Mog ? ), they say "Maddie". When they see her bag, they say "Maddie".
Mr McCann. Parfois ils disent “we love Madeleine”, et nous on répond “we love Madeleine”. Au pire, quand ils demanderont où elle est, nous dirons qu’elle a disparu ou qu’elle est perdue. C’est l’idée que nous devrons leur suggérer. On le fera quand ils seront prêts. Par exemple, ils ne comprennent pas quand ils voient Madeleine à la télévision. D’ailleurs nous ne l’allumons plus.
Sometimes they say "we love Madeleine" and we answer "we love Madeleine". At worst, when they ask where she is, we'll say she has disappeared or that she's lost. This is the idea that we'll have to suggest them. We'll do it when they're ready. For example, they don't understand when they see Madeleine on TV. Besides we don't switch it on.
Once I mentioned this case to a British friend of mine and she made me repeat when I said "Madeleine", then she said "she's known here as Maddie, nobody calls her Madeleine".