I got asked the question last week from DS4 and despite a bit of hedging and asking what he thought he kept saying "but is Santa really real. Tell me mum". So in the end I said no he is not but explained that lots of kids believe he is and if we join in and pretend he is there will be more presents under the tree. He was happy with that. This week he is talking about putting carrots out for the reindeer so the magic lives on...(phew)